Codecs & Media

DTMF (RFC 4733 / RFC 2833)

DTMF (Dual-Tone Multi-Frequency) is the touch-tone signaling used in IVRs, conference bridges, and call menus. RFC 4733 (formerly RFC 2833) defines how to carry DTMF events out-of-band in RTP rather than as tones inside the audio codec.

Three DTMF transports

1. In-band (audio): the actual DTMF tone is encoded by the codec. Works on G.711 (lossless enough). Fails on G.729 / Opus — the codec mangles the tone.
2. RFC 4733 (RTP events, recommended): a special RTP payload-type carries event codes (0-15 = digits 0-9, *, #, A-D) with redundant retransmissions. Works on any codec. This is what you want.
3. SIP INFO: sends DTMF as a SIP signaling message. Slower, signaling overhead, but useful when the media path can't be trusted (one-way calls, weird NATs).

SDP advertisement

m=audio 16384 RTP/AVP 0 8 101
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16

Common bug

If your IVR misses every other digit or hears 'pulses', you almost certainly have a DTMF mode mismatch. Force RFC 4733 on both sides and disable in-band.

Related terms