Topic summary
NPA code
Extracted from the Wikipedia article North American Numbering Plan.
Interchangeable NPA codes
In 1995, the North American Numbering Plan Administrator eliminated the requirement that the middle digit of an area code had to be either 0 or 1, implementing fully interchangeable NPA and central office codes, that had already been anticipated since the 1960s, when interchangeable central office codes were sanctioned.