Publicly accessible reverse telephone directories may be provided as part of the standard directory services from the telecommunications carrier in some countries. Some forms of city directories provide this form of lookup for listed services by phone number, along with address cross-referencing. As such, these directories are restricted to internal use only. Reverse telephone directories are used by law enforcement and other emergency services in order to determine the origin of any request for assistance, however these systems include both publicly accessible (listed) and private (unlisted) services. However, unlike a standard telephone directory, where the user uses customer's details (such as name and address) in order to retrieve the telephone number of that person or business, a reverse telephone directory allows users to search by a telephone service number in order to retrieve the customer details for that service. A reverse telephone directory (also known as a gray pages directory, criss-cross directory or reverse phone lookup) is a collection of telephone numbers and associated customer details.

Reverse yellow pages