Nunn, W. H. (1952). "Nationwide Numbering Plan". Bell System Technical Journal. 31 (5): 851–9. doi: 10.1002/j.1538-7305.1952.tb01412.x. Government of Canada, Public Services and Procurement Canada (2019-12-06). "To Drop or Not to Drop Parentheses in Telephone Numbers – Favourite Articles – Writing Tools – Resources of the Language Portal of Canada – Languages – Canadian identity and society – Culture, history and sport – Canada.ca". www.noslangues-ourlanguages.gc.ca . Retrieved 2022-02-10.

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The telephone exchanges—in the Bell System they were officially called central offices—became local exchange points in the nationwide system. Each of them was also assigned a three-digit number unique within its NPA. The combination of NPA code and central office code served as a destination routing code for use by operators to reach any central office through the switching network. [22] Due to the numerical structure of the numbering system, each NPA was technically limited to 540 central offices. [23] In October 1947, AT&T published a new nationwide numbering plan in coordination with the independent telephone operators. The plan divided most of North America into eighty-six numbering plan areas (NPAs). Each NPA was assigned a unique three-digit code, typically called NPA code or simply area code. These codes were first used in Operator Toll Dialing by long-distance operators in establishing calls via trunks between toll offices. The goal of automatic service required additional technical advancements in the latest generation of toll-switching systems, completed by the early 1950s, and installation of new toll-switching systems in most numbering plan areas. The first customer-dialed direct call using an area code was made on November 10, 1951, from Englewood, New Jersey, to Alameda, California. [6] Direct distance dialing (DDD) was subsequently introduced across the country. By the early 1960s, DDD had become commonplace in cities and most towns in the United States and Canada. By 1967, the number of assigned area codes had grown to 129. [7]

As the numbering plan grew in the 1960s under all-number calling, plan administrators at AT&T identified that by c. 1973 some of the largest area codes in urban centers might run out of central office prefixes to install more individual access lines. For relief in these cases, they finally removed the requirement that the middle digit of the central office code could not be 0 or 1. This resulted in the format of interchangeable central office codes, N X X, where N= 2–9 and X= 0–9. The first cities that required this action, in 1974, were the cities of Los Angeles with area code 213 and New York with 212. This change also required modification of the local dialing procedures to distinguish local calls from long-distance calls with area codes. All-number calling was a telephone numbering plan introduced in 1958, [33] that converted telephone numbers with exchange names to a numeric representation of seven digits. a b c "PL-418: Introduction of NPA 721 (Sint Maarten)" (PDF). North American Numbering Plan Administration. 2011-01-05. Archived from the original (PDF) on 2012-09-07 . Retrieved 2011-08-08. Updated by: "PL-423: Updated Information - Introduction of NPA 721 (Sint Maarten)" (PDF). North American Numbering Plan Administration. 2011-07-27. Archived (PDF) from the original on 2019-01-08 . Retrieved 2011-08-08. The Glenn Miller tune PEnnsylvania 6-5000 refers to telephone number PE6-5000, a number in service at the Hotel Pennsylvania (212 736-5000) in New York City until the hotel's closing in 2020. 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. Using 0 or 1 as the first digit of an area code or central office code 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.

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Of all states or territories, the U.S. state of California has the largest number of area codes assigned, followed by Texas, Florida and New York, while most countries of the Caribbean use only one. [57] 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. For some Pacific islands, the NANPA area code is the same as the country code that was discontinued upon membership in the NANP.The concepts underlying the NANP were originally devised in the 1940s by the American Telephone and Telegraph Company (AT&T) for the Bell System and the independent telephone companies in North America in Operator Toll Dialing. The first step was to unify the diverse local telephone numbering plans that had been established in the preceding decades, with the goal to speed call completion times and lower the costs for long-distance calling, by reducing manual labor by switchboard operators. Eventually, it prepared the continent for direct-dialing of long-distance calls by customers, first possible in 1951 and expanded across the nation in the decades following. AT&T continued to administer the continental numbering plan and the technical infrastructure until the breakup of the Bell System, when administration was delegated to the North American Numbering Plan Administrator (NANPA), a service that has been procured from the private sector by the Federal Communications Commission (FCC) in the United States. Each participating country forms a regulatory authority that has plenary control over local numbering resources. [1] The FCC also serves as the U.S. regulator. Canadian numbering decisions are made by the Canadian Numbering Administration Consortium. [2] 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. However, 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. The original plan of 1947 had been projected to be usable beyond the year 2000. However, by the late 1950s it became apparent that it would be outgrown by about 1975. [34] The limitations for the usable leading digits of central office codes, imposed by using common names for central office names, and their leading two characters as guides for customer dialing could no longer be maintained when opening new central offices. By 1962 it was forecast that in 1985 the number of telephones in the nation would equal its population of 280million and increase to 600million telephones for 340million people in 2000. [30] As a result, a few North American telephone administrations, notably New York Telephone Co., first introduced letter combinations that could not be linked to a familiar pronounceable central office name. Finally, they sought the elimination of central office names and letter codes, and introduced all-number calling (ANC). The toll-free telephone numbers in NPA 800, 888, 877, 866, 855, 844, and 833 have been portable through the RespOrg system since 1993. [54] Toll charges [ edit ] The Telecommunications Act of 1996 ( 47U.S.C. §251 (b)(2)) authorizes the Federal Communications Commission (FCC) to require all local exchange carriers (LECs) to offer local number portability. [50] The FCC regulations were enacted on June 27, 1996, with changes to take effect in the one hundred largest Metropolitan Statistical Areas by October 1, 1997, and elsewhere by December 31, 1998. [51]