History
The switchhook is the device that senses whether the handset or receiver is in its cradle. The term "flash" originated from the cord circuit of the early telephone switchboard that telephone company operators used to connect calls. The calling party and called party each had an indicator light on the cord circuit. When the subscriber cycled the telephone on-hook/off-hook, the light would flash. Actors in old movies often demonstrate the method, seeking the operator's attention. The user does a tap-tap-tap. When an operator comes on the line, the actor says, "Hello? Operator? We've been cut off." Then the operator attempts to reestablish the connection. The flashing light of this equipment of bygone days is the origin of the phrase "flashing the switchhook".
Centrex telephones added a hook-flash button in the 1960s after some users incorrectly attempted the attendant-recall function.
Read more about this topic: Hook Flash
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