Ringback signal
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In telephony, a ringback signal, usually consisting of an audio tone interrupted at a slow rate, provided to a caller to indicate that the called-party instrument is receiving a ringing signal. This signal may be generated by the called-party servicing switch or by the calling-party switch. A ringing signal returned to a caller to indicate that one of the types of delayed automatic calling is now ringing the called party.
Source: From Federal Standard 1037C and from MIL-STD-188