A ringback number is a telephone number for a telephone line that automatically calls the line that the call was placed from, after the caller has hung up. The typical use of this facility is by telephone company technicians for testing a new installation or for trouble-shooting.
Operation
Ringback testing is an acceptance testing procedure conducted by telephone installers to verify the quality of customer premises wiring to prevent network damage from faulty equipment or installation. The test consists of calling a reverting calling telephone number (ringback number) or a vertical service code with a standard telephone. Upon answering the call, the exchange plays a unique signal tone as a signal for the installer to hangup the telephone. The exchange then places a reverting call to the originating line, causing the telephone to ring. When answered, the exchange plays a verification tone. The verification tone is issued so that telephone subscribers cannot easily use the ringback system as an intercom between multiple stations in a residence on the same line by taking the stations off-hook.
Ringback numbers are typically not listed or communicated to subscribers.