Interchangeable NPA and central office codes

In the North American Numbering Plan (NANP), interchangeable NPA and central office codes constituted a change in numbering plan design and policy, to mitigate exhaustion of the numbering resources of the ten-digit telephone numbers used in the closed numbering plan of the NANP.

Interchangeable numbering plan area (NPA) codes and central office codes are codes that permit any of the ten numerals in the middle position of the area code and the central office code, which both are three-digit numbers. This change in numbering format was implemented first for central office codes by 1973, which eliminated the restriction in the middle digit (2 to 9) to also permit 0 and 1. The middle position of the area code could only be 0 and 1. In 1995, this restriction for area codes was lifted as well, creating interchangeable NPA codes.