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 digit broadly indicates the type of number. Numbers starting with 0 are for domestic calls: 0A (01-09) for landlines, 0AB0 (020, 050, 060, 070, 080, 090) for mobile phones, IP phones, and M2M communications, and 0XY0 (0120, 0570, 0800) for service numbers. 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 serves as the first clue for assessment. Calls from 050 numbers have a relatively higher chance of being sales or scam calls, and calling back 0570 numbers incurs charges. International numbers starting with + may be one-ring scams, with calls from +2xx (Africa) and +6xx (Pacific Islands) requiring particular caution. With knowledge of prefixes, you can roughly estimate the nature of a number the moment you see the incoming call screen, enabling faster decisions about whether to answer. You can learn systematically through the phone number structure guide.