Phone number lookup is a general term for services that use a phone number to find information about the caller, such as business name, industry, reviews, and nuisance call report counts. It is used when deciding whether to return a call from an unknown number and is widely relied upon as the first step in combating nuisance calls.
Lookup services fall into three main categories. First, community-driven websites that aggregate user reports and reviews (such as Denwa). Each number accumulates reports like "sales call" or "suspected fraud," and they can be checked instantly for free. Second, smartphone apps (such as Whoscall) that link to caller ID databases and display the caller's identity in real time during an incoming call, allowing you to decide before answering. Third, NTT's 104 directory assistance, a forward-lookup service that finds a phone number from a name - it does not support reverse lookups.
Phone number lookup and reverse lookup are similar concepts but technically distinct. Phone number lookup broadly refers to any act of finding out "who" from a number, while reverse lookup specifically refers to the technical method of querying a phone directory database in reverse. In practice they are used almost interchangeably, but reverse lookup relies on landline registration data, so information is often unavailable for mobile phone or IP phone numbers.
From a privacy perspective, services that publish individuals' phone numbers and names without consent raise concerns. Under Japan's Act on the Protection of Personal Information, phone numbers that can identify an individual constitute personal information, and third-party disclosure generally requires consent. Community-driven services operate in a legal gray area by sharing "reputation" about numbers rather than personal data, but users should not take the information at face value and should cross-reference multiple sources. See Phone Number Database Usage for how to choose between services and How to Block Nuisance Calls for concrete steps after a lookup.