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On analog telephone lines with special services, a flash or register-recall signal is used to control functions on the public telephone exchange, PBX or VoIP ATA.
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 register was normally disconnected from the circuit once a call was setup.
In contemporary telephone systems, the functions of the register are carried out by software and computer hardware, but in previous generations of electromechanical exchanges, using technology such crossbar or Reed relay, the register was often a system of analog electronics or even relay logic.
The term "Flash" or "Hook Flash" is commonly used in North America, while in Europe a similar signal is referred to as a register-recall or more commonly "Recall" or simply the "R" button. These signals perform similar functions, but are not necessarily identical.