A reverse telephone directory (also known as a blackpages
directory or reverse phone lookup) is a collection of telephone numbers
and associated customer details. 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.
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. As such these directories are restricted to
internal use only.
Publicly accessible reverse telephone 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.
On 29 June 2001, a legal case Telstra Corporation Ltd v Desktop
Marketing Systems Pty Ltd  was heard in the Australian Federal Court.
The case was lodged following the release of a product called "Phonedisk"
created by Desktop Marketing Systems Pty Ltd. The investigation found that the
content of that product, which allowed the user to search by service number,
was almost identical in content to the publicly listed directories, even if
some of the content was listed in a different fashion. The court's findings
gave Telstra, the predominant carrier within Australia and the maintainer of
the publicly accessible White Pages (residential) and Yellow Pages
(commercial) directories, copyright over the content of these directories.
As a result, that product was removed from sale. Further, a web-based
reverse directory, called "Blackpages", created by 2600 Australia, also
terminated its service.