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 used to check where a missed call came from and to judge whether a number is likely a nuisance call.
Reverse lookup draws on three main data sources. First, NTT's landline subscriber database. Only subscribers who consented to directory listing are registered, so a number can be traced back to a name for businesses that appear in the directory, while the area code and local exchange code narrow down the region the number was assigned to. Second, the Ministry of Internal Affairs and Communications' Telecommunications Number Designation data, which publicly shows which carrier holds each number block, giving a good indication 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.
NTT's 104 directory assistance was a forward-lookup service (name to number) and never supported reverse lookup, and it closed on 31 March 2026 in any case. For reverse lookups, web services and smartphone apps are the practical option. See How to Use Spam Call Databases for choosing between services and How to Look Up Area Codes for identifying regions from number blocks.