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DTMF guide: how phone keypad tones work in VoIP
DTMF is the signalling used by telephone keypad digits, *, and #. In VoIP, tones can be carried inside the audio, as RTP telephone-event packets, or as SIP messages.
The three common methods
| Method | How it works | Strengths and weaknesses |
|---|---|---|
| In-band audio | Tone is carried as ordinary audio | Simple, but compression, VAD, and transcoding can corrupt it |
| RTP telephone-event | Digit is sent as a distinct RTP event | Common and reliable across VoIP; negotiated in SDP |
| SIP INFO | Digit is sent in a SIP request | Can work in some systems, but support and routing are less consistent |
RTP telephone-event is often called RFC 2833, although the current specification is RFC 4733.
Troubleshooting an IVR that ignores digits
- Check the SDP for an agreed telephone-event payload type.
- Confirm the selected DTMF mode matches the PBX and carrier.
- Check whether a codec, VAD, or transcoder is altering in-band tones.
- Capture the call and confirm event packets are present when a digit is pressed.
- Test digits across both internal and external calls to isolate the boundary.
Related: voice codecs and packet captures.