VoIP glossary · Dual-Tone Multi-Frequency

What is DTMF?

DTMF (Dual-Tone Multi-Frequency) is the signalling used when someone presses a key on their phone, which IVRs, voicemail and payment systems use to read menu choices and PINs.

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DTMF explained

Each key produces a pair of tones. On VoIP these can be sent three ways: as audio within the call (in-band), as separate RTP "telephone-event" packets defined in RFC 2833/4733, or as SIP INFO messages. RFC 2833/4733 is the most reliable and is what most trunks expect.

DTMF problems usually come from a mismatch between methods, or from compressed codecs like G.729 distorting in-band tones. If callers report that "the menu doesn’t hear my button presses", check that both sides use RFC 2833 and that payload type 101 appears in the SDP.

Example

A caller presses 2 for billing. Their phone sends an RTP telephone-event packet for digit 2, and the IVR routes them to the billing queue.

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