This RFC defines two payload formats, one for carrying dual-tone multifrequency (DTMF) digits, other line and trunk signals, and a second one for general multi-frequency tones in RTP packets. Separate RTP payload formats are desirable since low-rate voice codecs cannot be guaranteed to reproduce these tone signals accurately enough for automatic recognition. Defining separate payload formats also permits higher redundancy while maintaining a low bit rate.

Read RFC 2833.