A prefix is a general term for the numbers assigned to the beginning of a phone number. It encompasses not only area codes (such as 03 and 06) but also mobile phone identification numbers (090, 080, 070), IP phone prefix 050, toll-free prefix 0120, and Navi Dial prefix 0570 - it is a higher-level concept covering the leading portion of all numbering systems.
It is important to understand the precise difference from area codes. Area codes are a term limited to regional identification for landline phones, carrying geographic information such as 03 (Tokyo's 23 wards) or 06 (Osaka City). Prefixes, on the other hand, are a comprehensive term referring to the leading portion of any phone number. 090 (mobile), 050 (IP phone), 0120 (toll-free), and 0570 (Navi Dial) are all prefixes but are not called area codes. In other words, area codes are a type of prefix, but prefixes are not limited to area codes.
Japanese phone number prefixes are organized so that the leading digits broadly indicate the type of number. Numbers beginning with 0 are for domestic use. Landline numbers are written as 0AB~J in the ministry's classification: the domestic prefix 0, followed by an area code of one to four digits and a local exchange code of one to four digits. Mobile numbers are 11 digits beginning with 090, 080, or 070; 050 belongs to IP phones; 020 is used for machine-to-machine communications and similar purposes; and 060 was added for mobile use when the Telecommunications Numbering Plan was amended in December 2024. The 0120, 0800, and 0570 ranges are for services such as toll-free and Navi Dial numbers, and they carry no regional meaning. Numbers starting with + are for international calls, with +81 being Japan's country code. Three-digit numbers starting with 1 (such as 110 and 119) are for emergency calls and special numbers.
In practice for nuisance call prevention, the prefix is the first thing to look at. Numbers such as 050 and 0570 are not tied to any region, so the number alone tells you nothing about where the caller is. Because the caller pays for 0570 calls, returning a call you had no reason to expect will cost you. International numbers starting with + are sometimes used for one-ring scams, so the safe course is not to call back countries you have no dealings with. Keep in mind, though, that caller ID spoofing can fabricate the number you see, so treating a prefix as proof of safety is the pitfall to avoid. The practical approach is to narrow down the type of number at a glance and, before calling back, check how the digits are arranged using the phone number structure guide.