A keypress can reach an IVR three ways: as in-band audio tones mixed into the call, as RFC 4733 telephone-event packets carried beside the audio, or as SIP INFO messages. Twilio's help centre states that Out of band RFC-2833 is supported. Out of band SIP INFO is not currently supported. Check the INVITE SDP for a=rtpmap:101 telephone-event/8000 before you change a single prompt.
What this covers
- The three ways a keypress travels, and what kills each one
- What Vapi, Retell AI, LiveKit and Twilio each send by default
- Why does my AI voice agent press a number and nothing happens?
- Eight real calls into a carrier IVR: what the send logs did and did not prove
- A dated example: LiveKit's DTMF sample-rate fix was merged, then reverted
This is a technical summary. The full guide — with the tables and worked examples — is on our site: *AI Voice Agent DTMF Not Working: Why Digits Miss*.
Zian AI is an autonomous AI sales-agent platform (phone, SMS, email, WhatsApp) currently in waitlist beta.
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