North American Numbering Plan - Us International Phone Number

The North American Numbering Plan (NANP) is a telephone numbering plan that encompasses 25 distinct regions in twenty countries primarily in North America, including the Caribbean and the U.S. territories. Not all North American countries participate in the NANP. Each participating country forms a regulatory authority that has plenary control over local numbering resources.

The NANP was originally devised in the 1940s by AT&T for the Bell System to unify the diverse local numbering plans that had been established in the preceding decades. Since shortly after the breakup of the Bell System, the numbering plan has been administered by the North American Numbering Plan Administration (NANPA), a service that is procured from the private sector by the Federal Communications Commission (FCC) in the United States. The FCC also serves as the US regulator. Canadian numbering decisions are made by the Canadian Numbering Administration Consortium.

The NANP divides the territories of its members into numbering plan areas (NPAs) which are encoded numerically with a three-digit telephone number prefix, commonly called the area code. Each telephone is assigned a seven-digit telephone number unique only within its respective plan area consisting of a three-digit central office code and a four-digit station number. The combination of an area code and the telephone number serves as a destination routing address in the public switched telephone network (PSTN). For international call routing, the NANP has been assigned the international calling code 1 by the International Telecommunications Union (ITU). The North American Numbering Plan conforms with ITU Recommendation E.164, which establishes an international numbering framework.

History

From its beginnings in 1876 and throughout the first part of the 20th century, the Bell System grew from essentially local or regional telephone systems. These systems expanded by growing their subscriber bases, as well as increasing their service areas by implementing additional local exchanges that were interconnected with tie trunks. It was the responsibility of each local administration to design telephone numbering plans that accommodated the local requirements and growth. As a result, the Bell System as a whole developed into an unorganized system of many differing local numbering systems. The diversity impeded the efficient operation and interconnection of exchanges into a nationwide system for long-distance telephone communication. By the 1940s, the Bell System set out to unify the various numbering plans in existence and developed the North American Numbering Plan as a unified, systematic approach to efficient long-distance service that eventually did not require the involvement of switchboard operators.

The new numbering plan divided the North American continent into regional service areas, called Numbering Plan Areas (NPA), primarily based on the boundaries of states and provinces. Each NPA was identified by a three-digit code number. The Numbering Plan Areas were created in accordance with principles that were deemed to maximize customer understanding and minimize the dialing effort, while reducing plant cost. The plan was designed so that any telephone within the service territory was identified by a unique 10-digit destination routing address. The leading part of this address was the area code (three digits), followed by a seven-digit subscriber number consisting of three digits for the central office exchange and four digits for the station or line number. Typical switching systems of the time were designed to serve up to 10,000 telephones, hence requiring four digits. The central office code was chosen such that it could be represented by the first two letters of the central office name according to a digit-to-letter mapping that was printed on the face of a rotary dial, by grouping a set of letters with the digits 2 through 9. Such letter translations, designed by W.G. Blauvelt in 1917, had been used in the Bell System in large metropolitan areas since the late 1910s. The network reorganization eventually resulted in a 2-letter 5-digit (2L-5N) representation of telephone numbers for every exchange in North America.

The new network design, completed in 1947, provided for 152 area codes, each with a capacity to serve 540 central offices. Originally only 86 area codes were assigned. New Jersey received the first area code in the new system, area code 201. The second area code, 202 was assigned to the District of Columbia. The allocation of area codes was readjusted as early as 1948 to account for inadequacies in some metropolitan areas. For example, Indiana area code 317 was split to provide a larger number pool in the Indiana suburbs of Chicago (area code 219). Area codes were first used by long-distance operators to establish long-distance calls between toll offices. The first customer-dialed direct call using area codes was made on November 10, 1951, from Englewood, New Jersey, to Alameda, California. Direct distance dialing (DDD) was subsequently introduced across the country and by the early 1960s most areas of the Bell System had been converted and it was commonplace in cities and most larger towns.

In the following decades, under an independent administration provided by Lockheed Martin IMS and later Neustar, the system grew to include the United States and its territories, Canada, Bermuda, and 17 nations of the Caribbean. At the request of the British Colonial Office, the numbering plan was first expanded to Bermuda and the British West Indies because of their historic telecommunications administration through Canada as parts of the British Empire, and their continued associations with Canada, especially during the years of the telegraph and the All Red Line system.

Not all North American countries participate in NANP, including Mexico, the Central American countries and some Caribbean countries (Cuba, Haiti, and the French Caribbean) are not part of the system. The only independent Spanish-speaking state in the plan is the Dominican Republic. Mexican participation was planned, but implementation stopped after two area codes were put into use (Mexico City and northwestern Mexico); these were withdrawn from use in 1991. Dutch-speaking Sint Maarten joined the NANP in September 2011.

Saint Pierre and Miquelon (+508) and Greenland (+299), both North American possessions of European Union nations, use non-NANP codes which are independent of their respective home countries (+33 France and +45 Denmark).

Until 1991, calls to some areas of Mexico from the United States and Canada were made using NANP area codes, but Mexico discontinued participation in the NANP in favor of an international format, using country code +52. Area code 905 (formerly Mexico City) was reassigned to a split of area code 416 (the Greater Toronto Area); area code 706 (formerly northwest Mexico) was reassigned to northern Georgia, surrounding the Atlanta region which retained 404; and area code 903, which also served a small portion of northern Mexico, was reclaimed and later reassigned to northeastern Texas when it split from area code 214.

Making International Phone Calls : How to Call the United States From the United Kingdom

Administration

The NANP is administered by the North American Numbering Plan Administration (NANPA). Today, this function is overseen by the Federal Communications Commission, which assumed the responsibility upon the breakup of the Bell System. The FCC solicits private sector contracts for the role of the administrator. Initially, the service was provided by a division of Lockheed Martin. In 1997, the contract was awarded to Neustar Inc. which was spun off from Lockheed for this purpose. In 2012, the contract was renewed until 2017.

Numbering system

A few of the most populous areas received area code assignments that took the shortest time to dial on rotary telephones: New York City was assigned 212, Los Angeles 213, Chicago 312, Detroit 313, Philadelphia 215. In the original plan the middle digit was 0 for plan areas that covered an entire state or province, while area codes with 1 in the second position were assigned to jurisdictions that were divided into multiple area codes.

At first, area codes had the form NYX, where N is any number 2â€"9, Y is 0 or 1, and X is any number 1â€"9 (if Y is 0) or any number 2â€"9 (if Y is 1). The restriction on N saves 0 for operator assistance, and 1 for signaling a long-distance call. The restriction on the second digit, limiting it to 0 or 1, was designed to help telephone equipment recognize the difference between a three-digit area code (with 0 or 1 as the second digit) and the three-digit central office prefix, which had avoided 0 or 1 for the second digit, because of restrictions in existing switching equipment. For example, when a caller dialed 202-555-0123, the switching equipment would recognize that 202 was an area code. If a caller were to dial 345-6789, the 4 would indicate the number to be local.

In the late 1980s and early 1990s, the NANPA implemented calling procedures that required all long-distance calls within an area code to be prefixed with 1 in an effort to make it possible to assign central office prefixes with 0 or 1 in the middle position. As it had nearly run out of area codes using the existing assignment methods, it allowed the assignment of area codes of the form N10, such as 210 in the San Antonio, Texas, area and 410 in eastern Maryland. Therefore, someone calling from San Jose to Los Angeles before the change would have dialed 213-555-0123 and after the change 1-213-555-0123, which permitted the use of 213 as an exchange prefix in the San Jose area.

The NANP number format may be summarized in the notation NPA-NXX-xxxx:

For example:

• 234-235-5678 is valid
• 234-911-5678 is invalid, because the exchange code cannot be in the form N11. (For non-geographic area codes like +1-800, 9-1-1 remains invalid, but other N-1-1 codes are valid exchange prefixes)
• 314-159-2653 is invalid, because the exchange code must begin with 2-9.
• 123-234-5678 is invalid, because NPA cannot begin with 0 or 1
• 281-234-5678 is valid.

The country calling code for the NANP is +1. In international format, an NANP number should be listed thus: +13015550100 (example using the original area code for Maryland). The trunk prefix code for direct-dialed long-distance in the NANP is also 1.

Each three-digit area code may contain up to 7,919,900 unique phone numbers:

• NXX may begin only with the digits [2â€"9], providing a base of 8 million numbers: ( 8 x 100 x 10000 ) .
• However, the last two digits of NXX cannot both be 1, to avoid confusion with the N11 codes.
• Despite the widespread usage of NXX 555 for fictional telephone numbers â€" see 555 (telephone number) â€" today, the only such numbers specifically reserved for fictional use are 555-0100 through 555-0199, with the remaining 555 numbers released for actual assignment as information numbers (subtract 100).
• In individual geographic area codes, several other NXX prefixes are generally not assigned: the home area code(s), adjacent domestic area codes and overlays, area codes reserved for future relief nearby, industry testing codes (generally NXX 958 and 959) and special service codes (such as NXX 950 and 976).

Various office codes in certain plan areas are deliberately not issued, for example, numbers 212718-xxxx, where 212 and 718 are both New York City area codes, are typically avoided to prevent confusion between an area code and a similarly numbered local exchange in the same region. 958-xxxx and 959-xxxx are usually test numbers. Inserting 0 or 1 as the first digit of an area code or seven-digit local number is invalid, as is a 9 as the middle digit of an area code; these are trunk prefixes or reserved for North American Numbering Plan expansion. Lists of exchanges in an individual area code (posted by CNAC in Canada, NANP in the United States) all list various prefixes as deliberately not issued.

Number portability

The Telecommunications Act of 1996 (47 U.S.C. § 251 (b)(2)) authorizes the Federal Communications Commission (FCC) to require all local exchange carriers (LECs) to offer local number portability. The FCC regulations were enacted on June 27, 1996 and changes were to take effect in the 100 largest Metropolitan Statistical Areas by October 1, 1997 and elsewhere by December 31, 1998. Toll-free telephone numbers (area codes 800, 888, 877, 866, 855 and 844) have been portable through the RespOrg system since 1993. The North American Numbering Council (NANC) was directed to select the Local Number Portability Administrators (LNPAs), also referred to as the Number Portability Administrator, akin to the North American Numbering Plan Administrator (NANPA).

Expansion

Canada and the United States have experienced rapid growth in the number of area codes, particularly between 1990 and 2005. There are two main reasons for this. First, there is the increasing demand for telephone services (particularly resulting from widescale adoption of fax, modem, and mobile phone communications).

The second and more important reason is the telecom regulation of local telephone service in the United States beginning in the early to mid-1990s. At that time, the Federal Communications Commission began allowing telecommunication companies to compete with the incumbent local exchange carrier (usually by forcing the existing sole service provider to lease infrastructure to other local providers who then resold the service to consumers). However, because of the original design of the numbering plan and telephone switching network that assumed only a single provider, number allocations had to be made in 10,000-number blocks.

Thus, whenever a new local service provider entered a given market, it would be allocated 10,000 numbers by default, even if the provider only obtained a few customers. As more companies requested numbering allocations, many area codes were faced with the problem of exhausting their number supply; in telecom jargon, such an area is "in jeopardy." Many of the new telecom ventures were not successful. While the number of area codes increased rapidly, this did not always translate to a much larger number of telephone subscribers as large blocks of numbers remained unassigned because of the 10,000-number block allocation methodology. When these telecom ventures were merged or terminated, their blocks transferred to the successors or remained unused. No regulatory mechanism existed to reclaim and reassign these underutilized blocks.

In general, area codes are added by two methods, splits and overlays. Splits were implemented by dividing a plan area into two or more regions, one of which retained the existing area code and the other areas receiving a new code. In an overlay, multiple codes are assigned to the same geographical area, obviating the need for any renumbering of existing services. Subtle variations of these techniques have been used as well, such as dedicated overlays, in which the new code is reserved for a particular type of service, such as cellular phones and pagers, and concentrated overlays, in which a part of the area retained a single code while the rest of the region received an overlay code. The only service-specific overlay in the NANP was area code 917 (New York City) when it was first installed; such service-specific area code assignments were later prohibited by the Federal Communications Commission.

Most area codes of the form N10, originally reserved for AT&T's Teletypewriter eXchange (TWX) service, were transferred to Western Union in 1969 and were freed up for other use in 1981 after conversion to Telex II service was complete. The last of these, +1-610-, was assigned to Canada and liberated in 1992. These "new" area codes (along with codes formerly used as aliases to Mexican numbers) were used for telephone area code splits in the late 1980s and early 1990s, as all other area codes under the original plan had been consumed.

After the remaining valid area codes were used up by expansion, in 1995 the rapid increase in the need for more area codes (both splits and overlays) forced NANPA to allow the digits 2 through 8 to be used as a middle digit in new area code assignments, with 9 being reserved as a "last resort" for potential future expansion. At the same time, local exchanges were allowed to use 1 or 0 as a middle digit. The first area codes without a 1 or 0 as the middle digit were area code 334 in Alabama and area code 360 in Washington, which both began service on January 15, 1995. This was quickly followed by area code 520 serving Arizona on March 19, 1995.

Codes ending in double digits are reserved as easily recognizable codes (ERCs), to be used for special purposes such as toll-free numbers, personal 500 numbers, Canadian non-geographic area code 600, carrier-specific 700 numbers, and high-toll 900 numbers, rather than for geographic areas. Nevada was denied 777 for this reason; it received 775 instead when most of Nevada split from 702, which continues to serve the Las Vegas metropolitan area.

Area code splits and overlays

By 1995, many cities in the United States and Canada had more than one area code, either through splitting the city into different areas (splits) or having more than one code for the same area (overlays). For example, in Manhattan, subscribers' numbers had the NPA code 212, but two additional codesâ€"first 917 (which initially was exclusively for cellular phones and pagers until that idea was struck down in a Federal court), then 646â€"were also introduced. This means that the area code must be dialed, even for local calls. In other areas, 10-digit or 11-digit dialing is now required for all local calls. The transition to 10-digit dialing typically starts with a permissive dialing phase in which both 7-digit and 10-digit dialing is optional. During this period, the transition is heavily publicized. After a period of several months, the mandatory dialing phase is introduced, in which 7-digit dialing no longer works. Atlanta was the first U.S. city to require mandatory 10-digit dialing throughout the metropolitan area, roughly coinciding with the 1996 Summer Olympics held there. Atlanta was used as the test case not only because of its size, but also because it enjoyed the world's largest fiber optic network at the time (five times that of New York then), and it was home to BellSouth (now part of AT&T), then the Southeastern Regional Bell Operating Company.

• 7-digit dialing: NXX xxxx (NPA code not required)
• 10-digit dialing: NPA NXX xxxx
• 11-digit dialing: 1 NPA NXX xxxx (1 is the NANP trunk prefix for long distance circuits).

The overlap between area codes and exchange prefixes has occasionally produced confusion because the three digits may be the same for both. Nashua, New Hampshire, for example, has a local exchange code, (603) 888. However, 888 is also an area code for toll-free service. If somebody in Nashua means to call 1-888-555-0121, but forgets the trunk prefix 1, the user might actually dial the local number 603-888-5550. This, however, is generally not a problem in major metropolitan areas with overlapping area codes, which were mandated by the FCC to dial all ten digits for all local calls so as not to give new numbers or telecommunications providers a disadvantage.

Expansion issues

Depending on the techniques used for area code expansion, the effect on telephone users varies. In areas in which overlays were used, this generally avoids the need for converting telephone numbers, so existing directories, business records, letterheads, business cards, advertising, and "speed-dialing" settings can retain the same phone numbers, while the overlay is used for new number allocations. The primary impact on telephone users is the necessity of remembering and dialing 10- or 11-digit numbers when only 7-digit dialing was previously permissible.

Splitting instead of overlaying generally avoids the requirement for mandatory area-code dialing, but at the expense of having to convert a region to the new code. In addition to the requirements of updating records and directories to accommodate the new numbers, for efficient conversion this requires a period of "permissive dialing" in which the new and old codes are both allowed to work. Also, many splittings involved significant technical issues, especially when the area splittings occurred over boundaries other than phone network divisions.

As an extreme example of an area code splitting gone somewhat awry, in 1998 area code 612, which had covered the Minneapolis â€" Saint Paul Twin Cities, was split to create area code 651 for St. Paul and the eastern metropolitan area. The Minnesota Public Utilities Commission mandated that the new boundary exactly follow municipal boundaries (which were distinctly different from telephone exchange boundaries), and that all subscribers keep their 7-digit numbers. These two goals were directly at odds with the reason for the split (to generate additional phone numbers), and there were more than 40 exchanges whose territory straddled the new boundary. The result was prefixes duplicated in both area codes, which counteracted much of the benefit of the splitting, with only 200 of 700 prefixes in area 612 moving entirely to area 651. As a result, in less than two years area code 612 again exhausted its supply of phone numbers, and it underwent another three-way split in 2000, creating the new area codes 763 and 952. Again, the split followed political boundaries rather than rate center boundaries, resulting in additional split prefixes; a few numbers moved from 612 to 651 and then to 763 in less than two years.

Decrease in expansion rate

Recognizing that the proliferation of area codes was largely due to the telecom regulation act and the assignment of numbers in blocks of 10,000, the FCC instructed NANPA, by then administered by Neustar, to alleviate the numbering shortage. As a result, number pooling was piloted in 2001 as a system for allocating local numbers to carriers in blocks of 1,000 rather than 10,000. Because of the then design of the switched telephone network, this was a considerable technical obstacle. Number pooling was implemented with another technical obstacle, local number portability.

The program has been implemented in much of the United States by state regulators. A limited number of cities have also implemented rate center^* consolidation; fewer rate centers resulted in more efficient use of numbers, as carriers would reserve blocks of 1,000 or 10,000 numbers in each of multiple rate centers in the same area even if they had relatively few clients in the area. Together with aggressive reclamation of unused number blocks from telecom providers, number pooling has reduced the need for additional area codes, so that many previously designated area splits and overlays have been postponed indefinitely.

There is no number pooling in Canada. Number allocation remains highly inefficient as even the tiniest village is a rate center and every CLEC is assigned blocks of ten thousand numbers in every place it offers new local service. As a result, dialing seven digits even in remote locations like James Bay is more likely to produce an intercept message ("dial the area code") than an actual voice connection.

New area codes outside the United States and Canada

Before 1995, all NANP countries and territories outside the fifty United States, the District of Columbia, and Canada shared the area code 809. This included Puerto Rico and the U.S. Virgin Islands. Each has since been assigned one or more distinct numbering plan areas; code (809) is only used by the Dominican Republic. The United States pacific territories of the Northern Mariana Islands and Guam joined the NANP in 1997, and American Samoa became a NANP member in October 2004. The Dutch possession of Sint Maarten was originally scheduled to join the NANP on May 31, 2010, but the changeover was postponed to September 30, 2011.

Number size expansion

The NANP exhaust analysis estimates that the existing numbering system is sufficient until 2044, based on the assumption that a maximum of 667 NPAs continue to be available and that on the average 4400 central office codes are needed per year.

Various plans are discussed for expanding the numbering plan to provide relief after exhaustion. One plan being considered adds digit 1 or 0 either to the beginning or to the end of the area code or the beginning of the sever-digit subscriber number. This would require 11-digit dialing even for local calls between any two NANP numbers. In another proposal, existing codes would be changed to x9xx, e.g. San Francisco's 415 would become 4915. Other proposals include reallocating blocks of numbers assigned to smaller long distance carriers or unused reserved services.

NANP countries and territories

Of all states or territories, the US State of California has the largest number of area codes assigned, followed by Texas, Florida, and New York, while most countries of the Caribbean only use one. Many Caribbean codes were assigned based on alphabetic abbreviations of the territory name, as indicated in the third column of the following table (Letter code). This follows the traditional letter assignments on telephone dials.

Dialing procedures

The structure of the North American Numbering Plan permits implementation of local dial plans in each plan area, depending on requirements. When multiple NPA codes serve an area in an overlay arrangement, ten-digit (10D) dialing is required. Seven-digit (7D) dialing may be permissible in areas with single area codes. Depending on the requirement of toll alerting, it may be necessary to prefix a telephone numbers with 1. The NANPA publishes dial plan information for individual area codes.

The standard dial plans in most cases are as follows:

Most areas allow permissive dialing of 10D or 1+10D even for calls that could be dialed as 7D. The number of digits dialed is unrelated to being a local call or a toll call when there is no toll alerting. Allowing 7D local dial across an area code boundary, which is uncommon today, requires central office code protection, locally if using toll alerting, across the entire area code otherwise, to avoid assignment of the same seven-digit number on both sides.

Most areas permit local calls as 1+10D except for Texas, Georgia, and some jurisdictions in Canada which require that landline callers know which numbers are local and which are toll, dialing 10D for local calls and 1+10D for all toll calls.

In almost all cases, domestic operator-assisted calls are dialed 0+10D.

Special numbers and codes

Some common special numbers in the North American system:

• 0 â€" Operator assistance
• 00 â€" Long-distance operator assistance (formerly 2-1-1)
• 011 â€" International Access Code. (For all destinations outside the NANP)
• 01 â€" International Access Code using operator assistance. (For all destinations outside the NANP)
• 101-xxxx â€" Used to select use of an alternative long-distance carrier
• 211 â€" Local community information or social services (in some cities)
• 311 â€" City government or non-emergency police matters
• 411 â€" Local telephone directory service (Some telephone companies provide national directory assistance)
• 511 â€" Traffic, road, and tourist information
• 611 â€" Telephone line repair service (formerly 4104), mobile telephone company customer service (formerly 811).
• 711 â€" Relay service for customers with hearing or speech disabilities.
• 811 â€" Dig safe pipe/cable location in the United States, non-urgent telehealth/teletriage services in Canada (formerly telephone company business office)
• 911 â€" Emergency telephone number â€" fire brigade, ambulance, police.
• 958-xxxx and 1-NPA-959-xxxx â€" Plant test numbers, such as automatic number announcement circuits. It was once common to reserve entire unused exchange prefixes or N11 numbers (4101 was ringback on many step-by-step switches), but these have largely moved to individual unpublished numbers within the standard 958-xxxx (local) or 959-xxxx (long-distance) plant test exchanges as numbers become scarce.
• 1-NPA-555-1212 â€" Non-local directory information (Canada and United States)

There are also special codes, such as:

• *51 and 1151: A history of unanswered calls on a telephone number, useful for those who are not Caller ID subscribers.
• *57 and 1157: Used to trace harassing, threatening, abusive, obscene, etc. phone calls, and keep results of trace at phone company.
• *66 and 1166: To keep retrying a busy-line (see also Called-party camp-on)
• *67 and 1167: Caller ID Block
• *69 and 1169: Call Return caller may press '1' to return call after hearing number
• *70 and 1170: Cancel call waiting on a call-by-call basis
• *71 and 1171: Three-way calling, which lets a person talk to people in two different locations at the same time.
• *74 and 1174: Speed dial, which allows someone to quickly dial any of eight frequently called numbers using a one-digit code, from any phone on their line. * *75 allows a total of 30 speed-call numbers with two digits.
• *82 and 1182: Releases Caller ID block on a call-by-call basis

Note: The four-digit numbers are not implemented in some areas. The codes prefixed with an asterisk (*) symbol are intended for use on Touch-Tone telephones, whereas the four-digit numbers prefixed 11xx are intended for use on rotary dial telephones, where the Touch-Tone * symbol is not available.

Not all NANP countries use the same codes. For example, the emergency telephone number is not always 911: Trinidad and Tobago and Dominica uses 999, as in the United Kingdom. The country of Barbados uses 211 for police force, 311 for fire, and 511 for ambulance, while Jamaica uses 114 for directory assistance, 119 for police force, and 110 for fire and ambulance services.

Despite its early importance as a share of the worldwide telephone system, few of the NANP's codes, such as 911, have been adopted outside the system. Determining that 911 requires unnecessary rotation time on rotary dial telephones, the European Union has adopted its own standardized number of 112, while countries in Asia and the rest of the world use a variety of other two- or three-digit emergency telephone number combinations. The 112 code is gaining prevalence because of its preprogrammed presence in mobile telephones that conform to the European GSM standard. The European Union and many other countries have chosen the International Telecommunication Union's 00 as their international access number instead of 011. The toll-free prefix 800 has been widely adopted elsewhere, including as the international toll-free country code. It is often preceded by a 0 rather than a 1 in many countries where 0 is the trunk prefix.

Alphabetic mnemonic system

Many dials on modern telephones in use in the NANP service areas maintain the tradition of alphabetic dialing. Usually each pushbutton from digit 2 to 9 also displays three letters, which is standardized in ISO 9995-8 and, in Europe, E.161. Historically, the letters Q and Z were omitted, although some modern telephones contain them. SMS-capable devices have all 26 letters. The alphabet is apportioned to the buttons as follows:

No letters are typically mapped to keys 1 and 0, although some corporate voicemail systems use Q and Z as 1, and some old telephones assigned the Z to the digit 0.

Originally, this scheme was used as a mnemonic device for telephone number prefixes. When telephone numbers in the United States were standardized in the mid-20th century to seven digits, the first two digits of the exchange prefix were expressed as letters rather than numbers, using the telephone exchange name. Before World War II, the largest cities used three letters and four or five numbers, while in most cities with customer dialing, phone numbers had only six digits (2L-4N). The prefix was a name, and the first two or three letters, usually shown in capital letters, were dialed. Later, the third letter, where implemented, was replaced by a digit, or an extra digit was added. This generally happened after World War II, although New York City converted already in 1930. The adoption of seven-digit local numbers (2L-5N) was chosen as the requirement for direct distance dialing and progressively deployed starting the late 1940s.

Thus, the famous Glenn Miller tune PEnnsylvania 6-5000 refers to telephone number PE6-5000, a number still in service at the Hotel Pennsylvania (212 736-5000) in New York. Similarly, the classic film BUtterfield 8 is set in the East Side of Manhattan between roughly 64th and 86th Streets, where the telephone prefixes include 288. In some works of fiction, phone numbers will begin with "KLondike 5" or KLamath 5, which translates to 555, an exchange that is reserved for information numbers in North America.

The letter system was phased out, beginning before 1965, although it persisted ten years later in some places. It was included in Bell of Pennsylvania directories until 1983. Even today, some businesses still display a 2L-5N number in advertisements, e.g., the Belvedere Construction Company in Detroit, Michigan not only still uses the 2L-5N format for its number (TYler 8-7100), it uses the format for the toll-free number (1-800-TY8-7100).

Despite the phasing out of the letter system otherwise, alphabetic phonewords remain as a commercial mnemonic gimmick, particularly for toll-free numbers. For example, one can dial 1-800-FLOWERS to send flowers to someone, or 1-800-DENTIST to find a local dentist. Sometimes, longer phonewords are used â€" for example one might be invited to give money to a public radio station by dialing 1-866-KPBS-GIVE. The number is eight digits long, but only the first seven need be dialed. If an eighth (or more) digit is dialed, the landline switching system will ignore it. Mobile and VoIP users may need to manually drop any numbers past the seventh digit as some mobile switching systems will not automatically ignore them, resulting in a failed call. Also, some users of smartphones can have difficulty dialing phonewords, as some of those devices do not have the apportioned letters on the keys used for dialing. This can be avoided by accompanying the use of phonewords with the actual numeric phone number, allowing users of such smartphones to dial using the numeric phone number. Some smartphones permit dialing phonewords by holding down a special function key, such as ALT in the case of the BlackBerry, while pressing another key on the qwerty pad.

In addition to commercial uses, alphabetic dialing has occasionally influenced the choice of regional area codes in the United States. For example, when area 423 (East Tennessee) was split in 1999, the region surrounding Knoxville was assigned area code 865, chosen to represent the word VOL (Volunteers), the nickname of Tennessee (The Volunteer State), as well as athletic teams at the University of Tennessee.

Several Caribbean area codes were chosen as an alphabetic abbreviation of the country name, which are indicated in the table of NANP regions.

The state of Nevada has previously attempted to obtain area code 777 (lucky 7s), but could not secure it.

Cellular services

The North American Numbering Plan does not set aside special non-geographic area codes exclusively for cellular phones. Only one regional exception exists in area code 600 in Canada. In many other national numbering plans, mobile services are assigned separate prefixes. Cell phone numbers in the NANP are allocated within each area code from special central office prefixes and calls to them are billed at the same rate as any other call. Consequently, the caller pays pricing model adopted in other countries, in which calls to cellular phones are charged at a higher nationwide rate, but incoming mobile calls are not charged to the mobile user, could not be implemented. Instead, North American cellular telephone subscribers are also generally charged to receive calls (subscriber pays). This has discouraged mobile users from publishing their number. However, price competition among carriers has reduced average price per minute for contract customers for both inbound and outbound calls, which compare favorably to those in caller-pays countries. Most users select bundle pricing plans that include an allotment of minutes expected to be used in a month. Of the four major national carriers in the United States, all four (AT&T, Sprint, T-Mobile, Verizon) offer free calling between mobile phones on the carrier's network, and Sprint also offers its customers free calling to mobile phones on other networks.

Some industry observers have blamed the subscriber-pays model as one of the main factors in the relatively low mobile phone penetration rate in the United States compared to that of Europe. In this model the convenience of the mobility is charged to the subscriber. Callers from outside the local-calling region of the assigned number, however, pay for a long-distance call, although domestic long-distance rates are generally lower than the rates in caller-pays systems. Conversely, an advantage of caller-pays is the relative absence of telemarketing and nuisance calls to mobile numbers. The integrated numbering plan also enables local number portability between fixed and wireless services within a region, allowing users to switch to mobile service while keeping their phone numbers.

The initial plan for overlays did allow for providing separate area codes for use by mobile devices, although these were still assigned to a specific geographical area, and were charged at the same rate as other area codes. Initially, the area code 917 for New York City was specifically assigned for this purpose within the boroughs; however, a Federal court struck this down and banned the use of an area code for a specific telephony purpose. Since mobile telephony is expanding faster than landline use, new area codes typically have a disproportionately large fraction of mobile and nomadic numbers, although landline and other services rapidly follow and local network portability can blur these distinctions.

The experience of Hurricane Katrina and similar events revealed a possible disadvantage of the methods employed in the geographic assignment of cellular numbers. Many mobile phone users could not be reached, even when they were far from the stricken areas, because the routing of calls to their phones depended on equipment in the affected area. They could make calls but not receive them.

The use of geographic numbers may also lead to tromboning; one can take a handset with a Vancouver number into St. John's and outbound calls to St. John's numbers while in that city will be local, but incoming calls must make the cross-country trip to Vancouver and back. This adds costs for subscribers, as an 8,000 km cross-country call (as a worst case) incurs long-distance tolls in both directions. AMPS subscribers used to be provided with a local number (such as 1-NPA-NXX-ROAM) in each city, allowing them to be reached by dialing that number plus the ten-digit mobile telephone number; this is no longer supported.

Toll charges

Calls between different countries and territories of the NANP are not typically charged as domestic calls. However, calls between the United States and Canada are considered domestic, but are typically charged at higher rates than calls to most domestic areas. Call costs to other destinations in the NANP are charged at overseas rates even on flat-rate plans in the United States and Canada; for example, it generally costs more to call Bermuda from the United States than it does to call the UK or Japan, even though the dialing format is the same as the domestic format. Similarly, calls from Bermuda to U.S. numbers (including toll-free 1-800) incur high international rates. This was because many of the island nations at the time implemented a plan of subsidizing the cost of local phone services by directly charging higher pricing levies on international long-distance services.

Because of these higher fees, scams had taken advantage of customers' unfamiliarity with pricing structure to call the legacy regional area code 809. Some scams lured customers from the United States and Canada into placing expensive calls to the Caribbean, by representing area code 809 as a regular domestic, low-cost, or toll-free call. The split of 809 (which formerly covered all of the Caribbean NANP points) into multiple new area codes created many new, unfamiliar prefixes which could be mistaken for U.S. or Canada domestic area codes but carried high tariffs. In various island nations, premium exchanges such as +1-876-HOT-, +1-876-WET- or +1-876-SEX- (where 876 is Jamaica) became a means to circumvent consumer-protection laws governing area code 900 or similar US-domestic premium numbers.

These scams are on the decline, with many of the Cable and Wireless service monopolies being opened up to competition, hence bringing rates down. Additionally, many Caribbean territories have implemented local government agencies to regulate telecommunications rates of providers.

Fictional telephone numbers

American television programs and films often use the exchange code 555 (KLamath 5 or KLondike 5 in older movies and shows) in fictitious telephone numbers, to prevent disturbing actual telephone subscribers if anyone is tempted to dial a telephone number seen on screen.

Occasionally, valid phone numbers are used contexts such as songs with varying intents and consequences. An example is the 1982 song "867-5309/Jenny" by Tommy Tutone, which is the cause of a large number of calls, although an Indianapolis plumbing company used both the tune and the number for advertising purposes.

Similarly, not all numbers beginning with 555 are fictional. For example, 555-1212 is the standard number for directory assistance. Only 555-0100 through 555-0199 are reserved for fictional use. Where used, these are normally information numbers; Canadian telephone companies briefly promoted 555-1313 as a pay-per-use "name that number" reverse lookup in the mid-1990s.