A reverse telephone lookup (also known as a reverse phone directory, greypages directory or criss cross directory) is a list of telephone numbers and associated customer information. Unlike a normal telephone search, where a customer’s details (such as name and address) are used to get a telephone number, a reverse telephone lookup allows users to search by telephone number in order to get the details about a customer for that service.

Reverse telephone lookups are used by law enforcement and other emergency services (such as 911) to find the location of requests for assistance, however these systems include both publicly available (listed) and private (unlisted) services. As a result, these directories are closed to general public use.

Reverse telephone lookup information is mostly provided by telecommunications companies. This information is also collected by special software that searches all available online directories and combines it into a special, compact report with details about the phone number owner.  This information includes the owner’s name, address, location and carrier.

Telephone companies in the United States have printed reverse phone directories for decades.  These directories were sent to phone companies, public libraries and law enforcement agencies. Starting in the early 1990s, businesses offered reverse telephone lookups for a small fee.  By the early 2000s, instant reverse phone number searches started being offered by online services.

In recent years, it has become very easy to do a reverse phone lookup.  Web-based services (such as PhoneNumberSearcher.com) offer reverse telephone searches so that consumers can search for unknown landline, unlisted and cellular phone numbers.  A caller’s number from caller ID can be entered on a web site to get instant information about the identity of an unknown caller.  This is helpful for knowing the identity of telemarketers, fund raisers and other companies who contact consumers by telephone. It is also helpful in tracking down and reporting prank phone calls.