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Collaboration, Voice & AV · Foundational

Dual-Tone Multi-Frequency (DTMF)

Touch-tone signaling in which each keypad digit is a pair of simultaneous tones, one low-group and one high-group frequency.

Definition

DTMF digits are used for dialing, IVR menu navigation, voicemail PIN entry and conference controls. Sent in-band they are reliable over G.711 but distorted by compressed codecs, so VoIP systems often relay them out-of-band using RFC 4733 named telephone events, SIP KPML, SIP NOTIFY, SIP INFO or H.245 alphanumeric/signal messages. A DTMF mismatch between call legs requires a media termination point to interwork methods.

Example

A caller on a G.729 call presses 1 for sales, and the gateway sends the digit as an RFC 4733 event rather than as audio.

Exam tip

One-way DTMF failure to voicemail or IVR usually points to a DTMF method mismatch needing an MTP or matched trunk settings.

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