History and background
The ten-digit telephone numbers of the NANP consist of a three-digit numbering plan area code (NPA code), written as the most-significant part of the telephone number, followed by the three-digit central office code, and the four-digit local line or station number. From 1947 to 1995, all NPA codes were distinguished with the digits ''0'' or ''1'' as the middle digit. This provided the traditional format ''N 0/1 X'', where ''N'' is any digit from ''2'' to ''9'', and ''X'' is any of the ten numerals. This format provided a set of 160 combinations, but only 144 were in use for geographic NPA codes. ''N00'' and ''N11'' were reserved for special service codes. In contrast, central office codes had the format ''N N X'', forbidding digits ''0'' and ''1'' in the middle position. These formats permitted direct distinction of NPA codes from central office prefixes by examination of the second digit dialed, which was necessary for automatic recognition ofInterchangeable central office codes
By 1971 the North American network had been prepared for operation with central office codes that permitted the digits ''0'' and ''1'' as the middle digit, i.e. with the number format ''NXX'' (less ''N11''), where ''N''=''2''–''9'', and ''X''=''0''–''9''.Bell Telephone Laboratories Staff, ''Engineering and Operations in the Bell System'', 2nd Edition (1984). p.118 Starting in c. 1974, large American cities (Los Angeles and New York City) began introducing office codes of this format, to keep up with demand while postponing area code splits by a few more years. In step-by-step offices, implementation usually required dialing an extra prefix to signal the dialing of a long-distance call with area code; ''112'' and ''115'' were early choices, but ''1'' became eventually common. In geographically small numbering plan areas that already were a single local calling area, it did not cause confusion in long-distance dialing, but in some cases, it became necessary to implement ten-digit dialing of long-distance calls within the same area code. This necessity was more common as interchangeable central office codes were introduced in more area codes. In the late 1980s, Bellcore, which administered the North American Numbering Plan, made it mandatory to implement interchangeable central office codes before area code relief would be approved. Thus, ten-digit dialing became more common in many area codes.Interchangeable NPA codes
By 1990, Bellcore determined the need for interchangeable NPA codes, which would be implemented no sooner than January 1, 1995. By that date, all telephone callers, even those in area codes that did not yet have interchangeable central office codes, would be required to dial all ten digits for long-distance calls, including such calls within the same area code. Canadian telephone companies made the change in Fall 1994 outside of area code 905, which had already implemented the requirement (before the split from 416) in 1990. The implementation of interchangeable area codes added 640 new codes to the numbering pool. The introduction of interchangeable codes required new technology for distinguishing a subscriber's intent of dialing seven or ten digits. Two methods were available, timing and extra prefixes. In the timing solution, the switching equipment would wait for more digits only for a critical ''time-out'' interval of three to five seconds. Such timing measurements were not necessary when no ambiguity existed from pretranslation. The preferred arrangement, over timing, was the ''prefix method'', in which the subscriber had to start dialing ten-digit calls with a ''0'' or ''1'' preceding the full telephone number. The preference was based on Bell System behavioral research, but technical aspect also favored this arrangement in some cases, in that it could shorten post-dial delay with new equipment. The first interchangeable NPA codes wereReferences
{{reflist Telephone numbers in the United States