North American Numbering Plan
The North American Numbering Plan is an integrated telephone numbering plan for twenty-five regions in twenty countries, primarily in North America and the Caribbean. This group is historically known as World Numbering Zone 1 and has the country code 1. Some North American countries, most notably Mexico, do not participate in the NANP.
The concepts of the NANP were devised in the 1940s in Operator Toll Dialing on the basis of the General Toll Switching Plan by the American Telephone and Telegraph Company for the Bell System and the independent telephone companies in North America. The first task was to unify the diverse local telephone numbering plans that had been established during the preceding decades, with the goal to speed call completion times and decrease 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, which expanded across the nation during the decades following. AT&T continued to administer the continental numbering plan and the technical infrastructure until the end of the Bell System, when operation was delegated to the North American Numbering Plan Administration, a service that has been procured from the private sector by the Federal Communications Commission in the United States. Each participating country forms a regulatory authority that has plenary control of local numbering resources. The FCC also serves as the U.S. regulator. Canadian numbering decisions are made by the Canadian Numbering Administration Consortium.
The NANP divides the territories of its members into numbering plan areas which are encoded numerically with a three-digit telephone number prefix, commonly termed the area code. Each telephone is assigned a seven-digit telephone number unique only within its respective numbering plan area. The telephone number consists 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. The North American Numbering Plan conforms with International Telecommunication Union Recommendation E.164, which establishes an international numbering framework.
History
From the Bell System's beginnings in 1876 and throughout the first part of the 20th century, telephone networks grew from essentially local or regional telephone systems. These systems expanded by growing their subscriber bases, as well as enlarging their service areas by implementing additional local exchanges that were interconnected with tie trunks. It was the responsibility of each local administration to devise telephone numbering plans that accommodated the local requirements and growth. As a result, the North American telephone service industry developed into an unorganized set 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 existing numbering plans to provide a unified, systematic concept for routing telephone calls across the nation, and to provide efficient long-distance service that eventually did not require the involvement of switchboard operators.In October 1947, AT&T published the first nationwide numbering plan in coordination with the independent telephone operators. The plan divided most of North America into eighty-six numbering plan areas. Each NPA was assigned a unique three-digit code, typically termed 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 advances 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. Direct distance dialing was introduced subsequently 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.
The status of the network of the 1960s was reflected by a new name used in technical documentation: North American Integrated Network. By 1975, the numbering plan was referred to as the North American Numbering Plan, resulting in the well-known initialism NANP, as other countries sought or considered joining the standardization.
Foreign expansion
Although Bermuda and the Caribbean islands had been assigned the area code 809 as early as 1958 by the administrators at AT&T, individual participating countries or territories had no autonomy over their numbering plan as they received centrally assigned central office prefixes that needed to be unique from those of other countries with the same area code. Regions in Mexico with high call volumes to and from the US were assigned functional area codes as early as 1963, for the purpose of call routing, but a nationwide system of participation in the NANP eventually failed.During the decades following, the NANP expanded to include all of the United States and its territories, Canada, Bermuda, and seventeen nations of the Caribbean.
Not all North American polities participate in the NANP. Exceptions include Mexico, Greenland, Saint Pierre and Miquelon, the Central American countries and some Caribbean countries. The only Spanish-speaking jurisdictions in the system are the Dominican Republic and Puerto Rico. Mexican participation was planned, but implementation stopped after three area codes had been used, and Mexico opted for an international numbering format, using country code 52. The area codes in use were subsequently withdrawn in 1991.
Sint Maarten, a Dutch Caribbean constituent, joined the NANP in 2011, being assigned area code 721. Sint Maarten shares the island of St. Martin with the French Collectivity of Saint Martin which, like the rest of the French Caribbean, is not a NANP member.
Administration
The NANP is administered by the North American Numbering Plan Administrator. This function is overseen by the Federal Communications Commission, which assumed the responsibility upon the federally mandated breakup of the Bell System. The FCC periodically solicits private sector contracts for the role of the administrator.Before the breakup of the Bell System, administration of the North American Numbering Plan was performed by AT&T's Central Services Organization. In 1984, this function was transferred to Bell Communications Research, a company created by the divestiture mandate to perform services for the newly created local exchange carriers. On January 19, 1998, the NANPA function was transferred to the IMS division of Lockheed Martin in Washington, D.C. In 1999, the contract was awarded to Neustar, a company created from Lockheed for this purpose. The contract was renewed in 2004, and again in 2012. On January 1, 2019, Somos assumed the NANPA function with a one-year bridge contract granted by the FCC with the goal of consolidating the NANPA function with the Pooling Administrator and identifying a long-term contractor. On December 1, 2020, Somos secured the $76 million contract for a term of eight years against one other bidder.
Numbering plan
The long-range vision of the architects of the North American Numbering Plan was a system by which telephone subscribers in the United States and Canada could themselves dial and establish a telephone call to any other subscriber without the assistance of switchboard operators. While the dialing of telephone calls by subscribers was common-place in many cities across the continent for local destinations, long-distance telephone calls had to be patched through manually by telephone operators at typically multiple intermediate toll offices using a system known since 1929 as the General Toll Switching Plan. The immediate goal for improvement in the time of call establishment was to provide technology for the originating toll operators to dial calls directly to the destination. This system was known as Operator Toll Dialing.Operator Toll Dialing required a nationwide telephone numbering plan that unified all local numbering plans into a consistent universal system. Local numbering plans, many of which required only four or five digits to be dialed, or even fewer in small communities, needed to be expanded. The goal was to preserve existing dialing patterns of the local telephone companies as much as possible.
Numbering plan areas and central offices
The new numbering plan divided the North American continent into regional service areas, termed numbering plan areas. The divisions conformed primarily to the jurisdictional boundaries of the U.S. states and the Canadian provinces, though some states or provinces needed to be divided into multiple areas. NPAs were created in accordance with principles deemed to maximize customer understanding and minimize dialing effort, while reducing plant cost. Each NPA was identified by a unique three-digit code number, termed the numbering plan area code, which was prefixed to the local telephone number when calling from one NPA to another. Calling within the same numbering plan area did not require dialing the area code, a feature today termed seven-digit dialing and today abandoned in numbering plan areas with multiple area codes.The telephone exchanges—in the Bell System they were officially termed 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. Due to the numerical structure of the numbering system, each NPA was technically limited to 540 central offices.
Although the limitation to 540 central offices required the most populous states to be divided into multiple NPAs, it was not the sole reason to subdivide a state. An important aspect was the existing infrastructure for call routing, which had developed during preceding decades, often independently of state boundaries. The rules of determining areas also attempted to avoid cutting across busy toll traffic routes, so that most toll traffic remained within an NPA, and outgoing traffic in one area would not be tributary to toll offices in an adjacent area. As a result, New York state was initially divided into five areas, the most of any state. Illinois, Ohio, Pennsylvania, and Texas were assigned four NPAs each, and California, Iowa, and Michigan received three. Six states and two provinces were divided into two NPAs.
Traditionally, central office switching systems were designed to serve as many as ten thousand subscriber numbers. Thus, subscribers were assigned four-digit line or station numbers. This rounded the total number of digits in a subscriber telephone number to ten: a three-digit area code, a three-digit central office code, and four digits for each line. This fixed format defined the North American Numbering Plan as a closed numbering plan, as opposed to developments in other countries where the number of digits was not fixed.
It was already common practice for decades that the digits 0 and 1 could not appear in the first two digits of the central office codes, because the system of using the first two letters of familiar names for central offices did not assign letters to these digits. The digit 0 was used for operator assistance, and 1, which is essentially a single pulse of loop interruption, was automatically ignored by most switching equipment of the time. Therefore, the 0/1 rule for the area code provided a convenient means to distinguish seven-digit dialing from ten-digit dialing.
The use of telephone exchange names as part of telephone numbers had been a well-established practice, and this was preserved for convenience and expediency in the new network design. The letter-to-digit translations were printed on the face of every rotary dial in the metropolitan areas, according to a scheme designed by W.G. Blauvelt in 1917, that had been used in the Bell System in large metropolitan areas since the early 1920s. The network reorganization standardized this system to using a two-letter and five-digit representation of telephone numbers in most exchanges in North America, or to using an equivalent all-numeric seven-digit numbering plan, as was practiced by some independent telephone companies.