Hook flash

The term "register-recall" in Europe refers to sending a discrete signal to alert the "register" — the logical system controlling a telephone exchange, that it should accept commands from the end user in the middle of a call.

The flash signal briefly disconnects the local loop circuit by momentarily depressing the hook switch or using a dedicated button.

The longer flash time programmed on a North American telephone, or a manual hook flash, may cause a European switch to clear the line, while a short pulse from a European phone may be ignored by a North American switching system.

Many modern telephones, sold across multiple markets, allow the end user to define the flash time in software or with a switch setting.

In European networks an “R” button is used in combination with touch tone digits to select various call handling functions.

On Centrex lines, PBX systems and VoIP ATAs a hook flash or the R-button is also used to perform call transfer (blind or with an enquiry) on analog extensions.

Many European telephones have a switch to configure the R button to perform this function instead of a timed break recall.

[3] A related service was often found on payphones in Europe and some other parts of the world where a Follow on Call (FC) button was often provided.

Centrex telephones added a hook-flash button in the 1960s after some users attempted the attendant-recall function incorrectly and disconnected their calls inadvertently.