Hook flash

Typical standard phone used with Centrex. Note the "Recall" button and the Message Waiting Lamp.

Hook Flash is a button on a telephone that simulates a quick off-hook/on-hook/off-hook cycle. Usually the functional equivalent of quickly hanging up and lifting the receiver on a middle 20th century cradle phone. Many modern telephones implement this function as a button. The feature was added to Centrex telephones in the 1960s when some users incorrectly attempted the attendant recall function.

On analog phone connections, this method can be used to signal the telephone exchange to do something. Usually it is used to indicate a request for voice conferencing. Another common use of hook flash is for switching to another incoming call with the call waiting service.

A traditional three-way calling example:

1. User picks up receiver handset on phone (causing the line to be off-hook, or active).
2. User hears a dialtone
3. User dials a number
4. User establishes phone conversation with other party.
5. User presses hook flash button (or quickly taps the sensor on the phone that indicates on-hook)
6. User usually hears a series of beeps, then another dialtone.
7. User dials another number
8. Once communications are established with this person, hook flash button is pressed again.
9. The phone switch at the phone company links the two conversations in software on its side.
10. Now three people are communicating at the same time.

The key differentiator here between this and other conferencing systems, is that the actual conference is happening at the phone switch, rather than at the customer premises.

Traditional conferencing involves having many lines at the customer site, and a more intelligent phone handling the multiplexing of the conversation internally.

History

The term "flash" originates in the cord circuit of the early telephone switchboard that telephone company operators used to connect calls. The calling party and called party each had a lamp 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." The switchhook is the device that senses whether the handset or receiver is in its cradle.