local exchange number; local exchange prefix
Telecommunications term for the part of a phone number that identifies the local exchange within an area code.
A phone number consists of an area code, a local exchange number, and a subscriber number.
市外局番 is the area code, while 市内局番 is the local exchange prefix within that area.
加入者番号 is the subscriber number that follows the local exchange prefix.
Compound of 市内 (within the city) + 局番 (exchange number).