Original North American area codes

The original North American area codes were established by the American Telephone and Telegraph Company in 1947. The assignment was in accord with the design of a uniform nationwide telephone numbering plan that supported the goal of dialing any telephone in the nation without involvement of operators at each routing step of a telephone call from origination location to its destination. The new technology had the aim of speeding the connecting times for long-distance calling by eliminating the intermediary telephone operators and reducing cost. It was initially designed and implemented for Operator Toll Dialing, in which operators at the origination point would dial the call as instructed by service subscribers, but had also the benefit of preparing the nation for Direct Distance Dialing by customers years later. The nationwide and continental application followed the demonstration of regional Operator Toll Dialing in Philadelphia during the World War II period.
The new numbering plan established a uniform destination addressing and call routing system for all telephone networks in North America which had become an essential public service. The project mandated the conversion of all local telephone numbers in the system to consist of a three-character central office code and a four-digit station number.
The initial "Nationwide Numbering Plan" of 1947 established eighty-six numbering plan areas that principally conformed to existing U.S. state and Canadian provincial boundaries, but fifteen states and provinces were subdivided further. Forty NPAs were mapped to entire states or provinces. Each NPA was identified by a three-digit area code used as a prefix to each local telephone number. The United States received seventy-seven area codes, and Canada nine. The initial system of numbering plan areas and area codes was expanded rapidly during the ensuing decades, and established the North American Numbering Plan.

Historical context

Early in the 20th century, the American and Canadian telephone industry had established criteria and circuits for sending telephone calls across the vast number of local telephone networks on the continent to permit users to call others in many remote places in both countries. By 1930, this resulted in the establishment of the General Toll Switching Plan, a systematic approach and network with technical specifications for routing calls between two major classes of routing centers, Regional Centers and Primary Outlets, as well as thousands of minor interchange points and tributaries. Calls were forwarded manually between stations by long-distance operators who used the method of ringdown to command remote operators to accept calls on behalf of customers. This required long call set-up times with several intermediate operators involved. For initiating a call, the originating party would typically have to hang up and be called back by an operator once the call was established.
The introduction of the first Western Electric No. 4 Crossbar Switching System in Philadelphia to commercial service, in August 1943, automated the process of forwarding telephone calls between regional switching points. For the Bell System this was the first test to let their long-distance operators dial calls directly to potentially distant telephones. While automatic switching decreased the connection times from as much as fifteen minutes to approximately two minutes for calls between far-away locations, each intermediate operator still had to determine special routing codes unique to their location for each call. To make a nationwide dialing network an efficient, practical reality, a uniform nationwide numbering plan was needed so that each telephone on the continent had a unique address that could be used independently from where a call originated. Such a methodology is called destination routing.
With this goal, AT&T developed a new framework during the early 1940s, termed Operator Toll Dialing, which was begun by the installation of a newly developed toll switching system in Philadelphia in 1943. In 1945, the American Telephone and Telegraph Company declared this effort a major post-war project for the Bell System, and proceeded with periodical communications to the general telecommunication industry via the Dial Interexchange Committee of the United States Independent Telephone Association, which disseminated the project's progress to its members via industry journals and conference contributions. The planning transitioned to implementation, when Ralph Mabbs presented the results in a talk at the Fiftieth Anniversary Meeting of the Independent Telephone Association, on October 14, 1947.
A fundamental requirement for the success of automated toll dialing was a new telephone numbering plan, which became known as the Nationwide Numbering Plan. This numbering plan accounted initially for seventy-seven area codes in the United States and nine in Canada. With the build-out and detailed analysis of existing technical infrastructure for toll dialing, the allocation maps needed to be modified in many states, adding numerous additional area codes during the next decade. By 1975, the numbering plan was known as the North American Numbering Plan, as efforts were in progress to expand the system beyond the United States and Canada.

Numbering plan requirements

Building a nationwide network in which any telephone could be dialed directly from anywhere in the country required a systematic numbering system that was easy to understand and communicate. Existing local telephone numbers varied greatly across the country, from two or three digits in very small communities, to seven in the large cities.
By the time the Bell Laboratories engineers began efforts to involve the broader industry bodies in 1945, local experiences had been studied in Pennsylvania and concepts had been developed for nationwide operator toll dialing. A crucial requirement was the conversion of all participating telephone networks to a universal numbering plan. In 1947, Ralph Mabbs recalled the specifications for this numbering plan as follows:

A distinctive telephone number for each telephone in the United States and Canada
The minimum number of digits which will provide for growth and new services
Minimum changes in customers' numbers
Minimum changes in local dialing practice
Least cost for equipment changes
Minimum reference by operators to bulletins and route guides to gain speed of service advantages
Provisions for operators to directly reach other operators at distant toll centers

Based on the precedent and experience with the large-city dial systems in the nation, the designers decided to direct all telephone companies in the nation to standardize the local telephone networks to seven-digit local telephone numbers before they could participate in operator toll dialing. This required few or no changes in the nation's largest cities, but in the smaller communities the shorter telephone numbers had to be padded with additional digits in a transparent, easily understandable manner, so that extra digits were not always needed when dialing other local subscribers. By 1955, AT&T disseminated a formal publication of network documentation, specifications, and recommendations to the telephone industry, entitled Notes on Nationwide Dialing.

Central office prefixes

Most automatic dial switching systems were designed since the early 1920s to provide service for up to ten-thousand subscriber lines for a complement of four digits in all telephone numbers. Each of these switching systems constituted a local telephone exchange, formally known as a central office.
Larger communities required multiple central offices to satisfy the service needs of their population. To accommodate more than ten-thousand telephone lines in a city for automatic dial service, extra digits were added to the telephone number, preceding the line number. Such extra digits identified a specific central office in the area to which the desired destination was connected. They served as routing codes to those central offices. In places without automatic routing between central offices, subscriber dialed the operator and asked for the destination at a specific central office, typically by the exchange name that was associated with each switching system. In automatic service regions, the exchange name, or central office name, could be dialed by the initial one, two, or three letters, as letters were arranged with the numeric equivalent on each telephone dial. Central office prefixes had already been used in the cities' dial systems since the 1920s, but only the largest of cities used three digits or letters.
This practice of dialing central office prefixes and users' familiarity of the system was preserved in the initial formulation of the new numbering plan, but was standardized to a format of using two letters and one digit in the prefix, resulting in the format 2L–5N for the subscriber telephone number.
For most cities, this conversion required the addition of extra digits or letters to the existing central office prefix. For example, the Atlantic City, New Jersey, telephone number 4-5876 was converted to AT4-5876 in the 1950s. Complete replacement of existing prefixes was necessary in the case of conflicts with another office in the state. Duplication of central office names, or an identical mapping of two different names to digits, was not uncommon. In practice, the conversion of the nation to this numbering plan took decades to accomplish and was not complete before the alphanumeric number format was abandoned during the 1960s in favor of all-number calling.
In addition to the central offices that provided the subscriber line for each telephone, the toll routing system included special switching facilities that exclusively routed long-distance calls between end offices. Each of these toll offices also received an assignment of a unique three-digit toll office code. To reach another operator in another central office or toll office, an operator dialed only the office code of the destination.