handle DTMF signaling across PSTN and WebRTC using RFC 4733 telephone-event
domain: datatracker.ietf.org · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Negotiate the telephone-event RTP payload type in SDP on both sides, with an fmtp listing supported event values (0–15 at minimum for DTMF digits).
On the sending side, generate telephone-event RTP packets instead of, or alongside, in-band audio tones when a user presses a key.
On the receiving/gateway side, decode telephone-event packets and, if bridging to PSTN, regenerate the actual DTMF/tone signal to inject into the TDM leg.
For WebRTC specifically, use RTCDTMFSender (built on this RFC) rather than hand-rolling telephone-event packets.
Verify interop by testing DTMF capture across the full call path (WebRTC leg + SIP trunk + PSTN termination), since mismatched payload type numbers are a common failure point.
Known gotchas
RFC 4733 obsoletes RFC 2833 by making event support negotiable rather than mandatory — a peer that doesn't explicitly negotiate the telephone-event payload type may fall back to in-band tones, which don't survive many compressed voice codecs cleanly.
The dynamic payload type number for telephone-event is negotiated per-call via SDP, not fixed — hardcoding a payload type instead of reading it from the SDP answer is a frequent source of 'DTMF doesn't work' bugs.
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