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Collaboration, Voice & AV · Intermediate

RFC 4733 Telephone Events

An RTP payload format that carries DTMF digits and other telephony events as named events in the media stream (formerly RFC 2833).

Definition

RFC 4733, which obsoletes RFC 2833, sends DTMF as RTP packets of a dynamic payload type, typically 101, that encode the event code, volume and duration rather than audio tones. It is negotiated in SDP with a telephone-event rtpmap and is the most common DTMF method on SIP trunks. Because it rides in RTP, it is called in-band signaling of out-of-band digits.

Example

SDP line a=rtpmap:101 telephone-event/8000 advertises RFC 2833/4733 DTMF support.

Exam tip

Many exams still call it RFC 2833; CUCM may need an MTP when one side supports only KPML and the other only RFC 2833.

Related terms