War dialing was a technique in the 1980s and '90s by which a computer would repeatedly dial a number (usually to a crowded modem pool) in an attempt to gain access immediately after another user had hung up.

An identical technique was sometimes used to get the first call for prizes in radio "call-in" shows, thus leading to the adoption of random "fifth caller," "seventeenth caller" etc. by radio stations to circumvent this practice.

See also: demon dialing, war driving