A subscriber number is the trailing portion of a phone number that uniquely identifies individual telephone lines (subscribers). For landlines, a single phone number consists of three elements: area code + local exchange number + subscriber number. For example, in 03-1234-5678, 03 is the area code, 1234 is the local exchange number, and 5678 is the subscriber number.
The digit count of subscriber numbers is not fixed but determined by the combination with the area code length. Japanese landline numbers follow a rule where "0 + area code + local exchange number + subscriber number = 10 digits" total. For Tokyo (03) and Osaka (06) with 2-digit area codes, it's 4-digit local exchange + 4-digit subscriber number. For regional cities with 4-digit area codes (like 0166), it's 2-digit local exchange + 4-digit subscriber number. The subscriber number itself is typically 4 digits, accommodating up to 10,000 lines (0000-9999) within a single local exchange number.
For mobile phones, the numbering system differs - the 8 digits following the 3-digit identifier (090/080/070) serve as the subscriber identification portion. Since mobile phones don't have the concept of area codes or local exchange numbers, they are not strictly called "subscriber numbers" but simply "numbers." Functionally, however, they serve the same role as landline subscriber numbers.
Number allocation is conducted under the Ministry of Internal Affairs' Telecommunications Numbering Plan, with local exchange number ranges designated for each carrier. Subscriber numbers are managed by each carrier and sequentially assigned from available numbers upon new subscriptions. In urban areas where number exhaustion is advancing, additional local exchange number allocations and numbering system revisions are becoming challenges. Review the overall numbering system in the phone number structure guide and the evolution of numbering in phone number digit history.