Interchangeable NPA and central office codes

Within the North American Numbering Plan (NANP), an interchangeable NPA code is a numbering plan area (NPA) code (area code) in the public switched telephone network (PSTN) that has digits other than 0 and 1 in the second (middle) position.

Area codes of the North American Numbering Plan are three-digit numbers that are unique to each designated numbering plan area of World Zone 1. 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 code. N00 and N11 were reserved for special service codes.

This format permitted direct distinction of NPA codes from central office prefixes, which were restricted from using 0 or 1 in the middle position of the three digit code, which was necessary for automatic recognition of seven-digit dialing for calls within the local calling area. Seven-digit dialing was standardized starting in the 1940s, in preparation of the Direct Distance Dialing (DDD) system, and was mostly complete in all but the most isolated or rural areas by 1970. DDD service for subscribers commenced in 1951 in Englewood, NJ.

In the early 1970s, large American cities started introducing interchangeable central office codes, which had a zero or one as the middle digit, to keep up with demand while postponing area code splits by a few more years. As this mostly affected geographically small area codes 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.

As of 1990, Bellcore determined that interchangeable NPA codes would be required and would be implemented no sooner than 1 January 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 first interchangeable NPA code was area code 334 in Alabama, followed by area code 360 in Washington state only one minute later, both entering mandatory use on January 15, 1995. Had ten-digit dialing not been made mandatory, customers in any other North American area code, trying to call the new 334 and 360 codes, would instead have had their dialing attempt misdirected, with the three extra digits ignored and the call instead routed to the 334 or 360 central office within their own area code.