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DTMF (Dual-Tone Multi-Frequency)

Updated 2026-09-30 · By the ZuteSIP team (ZuteSIP Inc.)

Short answer: DTMF (dual-tone multi-frequency) is the signaling that sends keypad digits during a call by combining two audio frequencies per key. It is how you enter extensions, PINs and menu choices on phone systems. On VoIP calls, DTMF is usually sent as RTP events or SIP INFO rather than as audio.

If menu key presses are ignored on a VoIP call, a DTMF mode mismatch is a common cause.

How it works

Each key maps to one low and one high tone. Compressed audio can distort these tones, so VoIP sends digits out of band: as RTP telephone-events (RFC 2833, updated by RFC 4733) or as SIP INFO messages carrying the digit.

DTMF methods compared

MethodHow digits are sentZuteSIP
In-band audioAs tones inside the audio, which compression can distortNot recommended
RFC 2833 (updated by RFC 4733)As RTP telephone-eventsSupported
SIP INFOAs SIP INFO messages carrying the digitSupported

How it applies to ZuteSIP

ZuteSIP supports DTMF via RFC 2833 (RTP events) or SIP INFO. In your softphone or PBX, set DTMF to RFC 2833 or SIP INFO rather than in-band audio when connecting to sip.zutesipdialer.com.

Related terms

  • IVR
  • G.711 codec
  • SIP
  • PBX

Frequently asked questions

Which DTMF mode should I pick?

RFC 2833 (often labeled RFC 4733 or telephone-event) is the most common choice; SIP INFO is also supported.

Why don't my key presses register?

Check that your client isn't set to in-band DTMF and that both sides agree on the mode.

Is RFC 2833 the same as RFC 4733?

RFC 4733 replaced RFC 2833 with the same RTP event format, so clients use the names interchangeably.

Related pages

Sources