RFC 2833 (RTP Payloads for DTMF Digits, Telephony Tones, and Telephony Signals) specifies an RTP payload format for carrying dual-tone multi frequency (DTMF) digits, and other line and trunk signals.
Complying with the RFC 2833 standard provides the following advantages:
Reduces the risk of low bit-rate vocoders such as G.723.1 rendering DTMF tones unintelligible to receiving gateways.
Makes it unnecessary for VoIP gateways to perform DTMF detection on incoming voice streams. Instead, applications can detect DTMF tones by waiting for specially formatted inband DTMF packets.
External Link: http://tools.ietf.org/html/rfc2833