Original North American area codes
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.
For the Bell System this was a beginning for the ability of their long-distance operators to dial calls directly to distant telephones.
In 1945, the American Telephone and Telegraph Company declared this effort a major post-war project for the Bell System,[5] and proceeded with periodical communications to the general telecommunication industry via the Dial Interexchange Committee of the United States Independent Telephone Association (USITA),[6] 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.
With the construction of technical infrastructure for automated toll dialing, the allocations needed to be changed in many states, adding numerous additional area codes during the next decade.
Building a nationwide network in which any telephone could be dialed directly required a systematic numbering system that was easy to understand and communicate.
By the time the Bell Laboratory engineers began efforts to involve the broader industry bodies in 1945, definite concepts had been developed for Operator Toll Dialing.
By 1955, AT&T disseminated a formal publication of network documentation, specifications, and recommendations to the telephone industry, entitled Notes on Nationwide Dialing.
Such extra digits were dialed when calling a telephone connected to another switching system in the same city or in a nearby community, and served as routing codes to those central offices.
This was the opportunity for distinction, but only when 0 or 1 were used in the second position, because switching systems already suppressed single loop interruption (corresponding to 1) automatically, and 0 was used to reach an operator or long-distance desk.
As network growth could not be predicted accurately over more than a few years, this pattern was abandoned by 1956, when call routing into New Jersey was divided between two toll centers.
Next to size, another important aspect was the existing infrastructure for call routing, which had developed during preceding decades independently of state or municipal boundaries.
[11][9] However, it was recognized already in 1930 that too little granularity in the allocation pattern introduced expensive traffic back-haul requirements, conceivably resulting in congestion of the routes to the centers.
Illinois, Ohio, Pennsylvania, and Texas were assigned four area codes each, and California, Iowa, and Michigan received three.
Such a pattern suggests that the designers intended to maintain the same degree of randomness in digits for the remaining, yet unassigned codes.
The red fields are the NPAs that hosted the Regional Centers for toll-switching established in the General Toll Switching Plan of 1929:[2] New York City (212), Los Angeles (213), Dallas (214), Chicago (312), St. Louis (314), and San Francisco (415) in the multi-NPA states, and in Denver (303) and Atlanta (404) in states with just a single area code each.
[7] For entering the destination codes and telephone numbers into newly designed machine-switching equipment, long-distance operators did not use the slow rotary dials, but a ten-button key set, operating at least twice as fast, which transmitted multi-frequency (MF) tone pulses over regular voice channels to the remote switching stations.
All existing toll switching offices, many still using direct-control (step-by-step) methods, had to be supplemented with components to permit MF signaling and automatic route selection.
[22] In 1950, southwest Missouri, with a new toll-center in Joplin, received area code 417, a change that provided more central offices in Kansas City.
[23] By 1951, a new Class-5 switching system, the No.5 Crossbar, had proven successful in Media, Pennsylvania, and automatic message accounting (AMA) became available for billing telephone calls.
[25] On November 10, 1951, the system commenced a customer trial of direct distance dialing (DDD) from this single location in the country.
While the first customer-dialed call using an area code was made in 1951, it took nearly fifteen years after the 1947 announcement of the new numbering plan that direct distance dialing was common in most cities.
In 1960, AT&T engineers, estimating that the capacity of the numbering plan would be exhausted by 1975,[28] prepared for the next major advance in the evolution of the network by eliminating central office names, and introducing all-number calling (ANC).
