A reverse cell phone number directory is also known as wireless greypages directory or reverse cell phone lookup which is a collection of cellular telephone numbers and associated customer details. However, unlike a standard phone number directory, where the user eneters a name in order to retrieve the telephone number of that person or business, a reverse phone number directory allows you to start search by a using wireless phone number number in order to retrieve the customer details such as first and last name, address and account history for that wireless phone number account.

Reverse cellular phone 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. As such, these directories are restricted to internal use only.

Publicly accessible reverse cell phone directories may be provided as part of the standard directory services from the telecommunications carrier in some countries. In other countries these directories are often created by phone phreakers by collecting the information available via the publicly accessible directories and then providing a search function which allows users to search by the telephone service details.