Hook 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). This action can signal the telephone exchange to do something. A common use of hook flash is to switch to another incoming call with the call waiting service. 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:

1. Pick up phone handset (causing the line to be off-hook).
2. Hear a dialtone
3. Dial the first number and greet the first party
4. Press the hook flash button (or quickly tap the on-hook sensor on the phone)
5. Hear a stutter dial tone (a series of beeps followed by another dialtone)
6. Dial the second number and greet the second party
7. Press the hook flash button again.

The second "flash" signals the Central Office Switch to link the two active conversations, so that all three parties are connected to the same logical telephone line.

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