About: All-number calling

All-number calling (ANC) is a telephone numbering plan that was introduced into the North American Numbering Plan by the Bell System in the United States starting in 1958 to replace the previous system of using a telephone exchange name as the first part of a telephone number. The plan prescribed the format of a telephone number assigned to subscriber telephones to consist of ten digits, composed from a three-digit area code, a three-digit central office code, and a four-digit station number. This increased the number of effectively available central office codes in each numbering plan area (NPA) from 540 to 792, thereby staving off the threat of exhausting the number pool, which was forecast to occur by the late 20th century.