Interchangeable NPA and central office codes

In North America, interchangeable NPA codes are area codes of the telephone network that do not have a 0 or 1 as the middle digit.

In North America, from 1947 to 1995, all numbering plan area (NPA) codes were distinguished with a 0 or 1 as the middle digit, while central office codes (the first three digits of a seven-digit local number) never had a 0 or 1 as the middle digit.

Consequently, as this seven-digit numbering was gradually implemented throughout the continent (being standard in all but the most isolated or rural areas by 1970), and Direct distance dialing was introduced to customers, the switching system 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 began to introduce "interchangeable central office codes", that 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 CO 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 CO codes before area code relief would be approved. Thus, 10-digit dialing began to come into use in more and more 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 CO codes, would be required to dial all ten digits for long distance calls, including such calls within the same area code. Canadian phone companies made the change in fall 1994 outside of the 416 and 905 areas, which had already implemented the requirement in 1990.

On 7 January 1995, area code 630 was activated in the Chicago area, the first interchangeable NPA code. Had ten-digit dialing not been made mandatory, customers in any other North American area code, trying to call the new Chicago area code 630, would instead have had their dialing attempt misdirected, with the three extra digits ignored and the call instead routed to the 630 central office within their own area code.