Hook flash

flashesRecallswitchhook flash
Hook flash or flash (known in the UK as "recall") is a button on a telephone that simulates quickly hanging up then picking up again (a quick off-hook/on-hook/off-hook cycle).wikipedia
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Call waiting

call-waiting
A common use of a hook flash is to switch to another incoming call with the call waiting service.
The subscriber may switch between calls, typically by using the hook flash signal.

Conference call

conference callingconference callsaudio conferencing
Another use is to indicate a request for voice conferencing, for example, a user may use a procedure like the following to initiate three-way calling:
Then the hook flash button (or recall button) is pressed and the other called party's phone number is dialed.

On- and Off-hook

off-hookon-hookon hook
Hook flash or flash (known in the UK as "recall") is a button on a telephone that simulates quickly hanging up then picking up again (a quick off-hook/on-hook/off-hook cycle). 1) Pick up phone handset (causing the line to be off-hook).
Hook flash

Telephone hook

switchhookhandset cradlehook switch
The switchhook is the device that senses whether the handset or receiver is in its cradle.
Pushing the switchhook quickly is termed a "hook flash".

Telephone

phonetelephonesLocal Telephone Service
Hook flash or flash (known in the UK as "recall") is a button on a telephone that simulates quickly hanging up then picking up again (a quick off-hook/on-hook/off-hook cycle).

Telephone exchange

exchangescentral officeexchange
This action can signal the telephone exchange to do something.

Business telephone system

PBXprivate branch exchangekey telephone system
In contrast to PBX conferencing systems, the two calls are joined at the Central Office switch, rather than at the customer premises PBX.

Payphone

pay phonepayphonespublic telephone
On payphones, this function is usually provided with a button labeled Follow-on call, which is mainly used for requesting another dial tone after finishing a call.

Handset

telephone handsethandset receiverphone receivers
1) Pick up phone handset (causing the line to be off-hook).

Cord circuit

junctors
The term "flash" originated from the cord circuit of the early telephone switchboard that telephone company operators used to connect calls.

Telephone switchboard

switchboardswitchboardscentral switchboard
The term "flash" originated from the cord circuit of the early telephone switchboard that telephone company operators used to connect calls.

Calling party

callercall originatorcall
The calling party and called party each had an indicator light on the cord circuit.

Called party

calleebe calledcall receiver
The calling party and called party each had an indicator light on the cord circuit.

Check engine light

indicator lampMalfunction indicator lampcheck engine" light
The calling party and called party each had an indicator light on the cord circuit.

Local loop

subscriber lineloopsubscriber loop
The calling party and called party each had an indicator light on the cord circuit.

Centrex

Centrex telephones added a hook-flash button in the 1960s, after some users incorrectly attempted the attendant-recall function.

Signaling (telecommunications)

signalsignalingsignalling
Line signaling is concerned with conveying information on the state of the line or channel, such as on-hook, off-hook (answer supervision and disconnect supervision, together referred to as supervision), ringing, and hook flash.

Ringback number

ringbackplant test numbers
When answering, the switch may play a low frequency signal tone, at which point the technician flashes the line (hangs up briefly) one or more times.

R (disambiguation)

R
R- or Recall-button. Infrequent button still on some phones today (2017), now mostly used to switch between two calls

Call transfer

transferCT
A call transfer is a telecommunications mechanism that enables a user to relocate an existing telephone call to another phone or attendant console, using a transfer button or a switchhook flash and dialing the required location.

Loop start

This signal is called flashing or hook flash, and is performed by interrupting the loop for a fraction of a second, typically at least 300 ms. The flash signal is longer than a rotary dial pulse, sometimes called a short flash, and is shorter than the time required for the on-hook condition.