Ringback tone

A ringback tone (RBT, alternatively ringing tone) is an audible indication that is heard by the originator of a telephone call while the destination being called is ringing. It is normally a repeated tone, designed to assure the calling party that the called party's line is ringing, although the ring-back tone may be out of sync with the ringing signal.

The ringback tone may be generated in the distant switch and transmitted in-band. In analog networks the caller could therefore monitor the quality of the complete voice path of the connection before the call was established. The distant switch also sends a message out-of-band indicating to the rest of the following network that the phone is ringing. Under strict implementation of the SS7 protocol and the CAMEL signaling system, the closest switch to the caller may be generating the ringback tone. In most public phone networks the ringback tone is not generated in the handset or by the local switch, as customized tones or voice announcements may be generated by the distant switch in place of a ringing signal.