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. This digit count is where practical work tends to break: an input field hard-coded as "4-digit exchange plus 4-digit subscriber number" will reject numbers from areas with a 4-digit area code (0166-XX-XXXX). Storing the 10 digits without hyphens and inserting the separators at display time, according to the length of the area code, is the safer approach.
For mobile phones, the numbering system differs: the 8 digits following the 3-digit identifier (090/080/070) serve as the subscriber identification portion. Because mobile numbers have no area code or local exchange segment, the three-part breakdown used for landlines does not map onto them directly. The role is the same, though: the trailing digits are what distinguish one contracted line from another.
Number allocation is conducted under the Ministry of Internal Affairs and Communications' 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. Where number exhaustion is a concern, the usual remedy is to designate additional local exchange numbers for the area so that more lines can be accommodated. Review the overall numbering system in the phone number structure guide and the evolution of numbering in phone number digit history.