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DTMF (Dual-Tone Multi-Frequency)

Updated 2026-10-23 · By the SMS Conecta team (SMSConecta Inc.)

Short answer: DTMF, or dual-tone multi-frequency signalling, carries keypad digits during a call by pairing two audio frequencies per key. It is the mechanism behind entering extensions, PINs and menu choices on a phone system. On VoIP calls, DTMF is typically carried as RTP events or SIP INFO rather than as plain audio.

When menu key presses get ignored on a VoIP call, a mismatched DTMF mode is usually the culprit.

How it works

Each key corresponds to one low tone and one high tone. Because compressed audio can distort those tones, VoIP instead sends digits out of band — as RTP telephone-events (RFC 2833, later updated by RFC 4733) or as SIP INFO messages carrying the digit directly.

Comparing DTMF methods

MethodHow the digits travelSMS Conecta
In-band audioSent as tones mixed into the audio, which compression can garbleNot advised
RFC 2833 (updated by RFC 4733)Sent as RTP telephone-eventsSupported
SIP INFOSent as SIP INFO messages carrying the digitSupported

How it applies to SMS Conecta

SMS Conecta handles DTMF through either RFC 2833 (RTP events) or SIP INFO. In a softphone or PBX, set DTMF to RFC 2833 or SIP INFO rather than in-band audio when connecting to sip.smsconecta.com.

Related terms

  • IVR
  • G.711 codec
  • SIP
  • PBX

Frequently asked questions

Which DTMF mode is the right choice?

RFC 2833 (sometimes labelled RFC 4733 or telephone-event) is the most common pick, though SIP INFO works just as well.

Why aren't my key presses registering?

Check that the client isn't configured for in-band DTMF, and confirm both ends agree on the same mode.

Are RFC 2833 and RFC 4733 the same standard?

RFC 4733 superseded RFC 2833 while keeping the same RTP event format, so the two names get used interchangeably.

Related pages

Sources