An area code is a regional identification number assigned to the beginning of a landline phone number. Under the Ministry of Internal Affairs and Communications' Telecommunications Numbering Plan, area codes are systematically assigned to municipalities across Japan. Since you can estimate the caller's region just by looking at the number, area codes are also useful for phone number lookups and identifying nuisance calls.
Area codes range from 2 to 5 digits, with larger cities receiving shorter codes. 03 (Tokyo's 23 wards) and 06 (Osaka City) are 2 digits, 045 (Yokohama) and 011 (Sapporo) are 3 digits, and 0166 (Asahikawa) is 4 digits. Shorter area codes leave more digits available for the local exchange number and subscriber number, allowing more phone numbers to be accommodated.
A commonly confused concept is the prefix, but area codes are a term limited to regional identification for landline phones. Mobile phone numbers starting with 090/080/070 and IP phone numbers starting with 050 are not area codes - they are managed under a separate telecommunications numbering system. Knowing how to look up area codes helps you quickly identify the region when receiving calls from unknown numbers.
One practical caution: do not judge call rates from the area code alone. Charges are set by the rate table of the carrier you have a contract with, so check your contract terms rather than how the number looks. For how the number itself is read, see phone number structure. Even after NTT's migration from PSTN to an IP network - carried out in stages from January 2024 and completed by January 2025 - the area code system has been kept in place, and nothing changes in how users dial. That said, the mapping between area codes and regions can change when the Telecommunications Numbering Plan is amended, so you can check which regions numbers belong to on the area code map.
When calling from a landline within the same area-code zone, the area code can be omitted and dialing can start from the local exchange number (calls from mobile phones always require the full number including the area code). Numbers starting with 03 (Tokyo's 23 wards) or 06 (Osaka City) are also read as a signal of a company's physical location, and are perceived differently from 050 numbers, which are not tied to any region.