Interchangeable NPA code

Within the North American Numbering Plan, an interchangeable NPA code is a numbering plan area (NPA) code (area code) of the telephone network that does not have the digits 0 or 1 in the second (middle) position.

From 1947 to 1995, all NPA codes were distinguished with a 0 or 1 as the middle digit, while central office codes, i.e. the first three digits of the seven-digit subscriber telephone number, never had 0 or 1 as the middle digit.

Seven-digit dialing was standardized starting in the 1940s, and was mostly complete in all but the most isolated or rural areas by 1970. Simultaneously, starting in 1951, Direct Distance Dialing (DDD) was introduced to customers. Switching systems could determine from the second digit dialed, or the second digit that followed the trunk code, whether the call was being dialed as seven digits (without an area code) or ten digits (with an area code).

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 10-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, 10-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 code to be activated was area code 334 in Alabama, followed one minute later by area code 360 in Washington state, 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.