Editing Ringback number (section)

==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.<ref name=":0">[https://cdn.loc.gov/service/ll/fedreg/fr050/fr050139/fr050139.pdf 50 FR 29391] ''[[Federal Register]],'' '''50''' (139), (19 July 1985). Seen 03 February 2021.</ref>

Ringback numbers are typically not listed or communicated to subscribers.<ref name=":0" />