Reverse lookup is a search method that takes a phone number as input and identifies the owner or business associated with it. While a standard phone directory looks up a number from a name (forward lookup), reverse lookup works in the opposite direction - hence the name. It is an essential tool for identifying missed callers and tracking down nuisance call sources.
Reverse lookup draws on three main data sources. First, NTT's landline subscriber database, which contains information for subscribers who have consented to directory listing, allowing the region and business to be identified from the area code and local exchange code. Second, the Ministry of Internal Affairs and Communications' Telecommunications Number Designation data, which publicly shows which carrier holds each number block, enabling identification of the number type (landline, mobile, IP phone). Third, community-driven web services and apps that aggregate user reports - phone number lookup sites fall into this category, accumulating reports such as "sales call" or "suspected fraud."
Reverse lookup has technical limitations. Mobile phone numbers are not listed in phone directories, so NTT's database cannot identify the owner. IP phone 050 numbers face the same limitation. Additionally, when caller ID spoofing is used, looking up the displayed number will not lead to the actual caller. Community-driven services help fill these gaps, but since accuracy depends on contributors, it is important to cross-reference multiple sources.
Note that NTT's 104 directory assistance is a forward-lookup service (name to number) and does not support reverse lookup. For reverse lookups, web services and smartphone apps are the practical option. See Phone Number Database Usage for choosing between services and How to Look Up Area Codes for identifying regions from number blocks.