Selcall (Selective Calling) transmits a burst of five in-band audio tones to initiate the conversation. In a simplex (one way at a time) system, the 5-tone just opens the speaker of the desired partner. In a repeater system, another CTCSS or tone-burst or 5-tone is needed to activate the repeater, depending on the systems design. If the called radio is within reach of the sender, it answers the incoming call with its stored receipt tone. Sometimes systems using Selcall are referred to as CCIR or ZVEI, specific tone encoding schemes used in Selcall systems. In the same way that a single CTCSS tone would be used on an entire group of radios, a single five-tone sequence is used in a group of radios. All radios also have their own private callnumber stored, to be reached for an individual conversation instead of a group call. In either way the radio speaker turns on as soon as the fifth tone of a valid sequence is decoded. In case of a group call, a short announcement tone is generated on the radios speaker. In case of a private call, the receipt tone is transmitted back to the sender and then the receive path is open. The speaker stays on until the carrier squelch detects that the carrier is no longer being received. At that point, the speaker mutes and the decoder resets. The receiver speaker turns off and remains muted until another valid five-tone sequence is decoded.

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