​DTMF (Dual-Tone Multi-Frequency) is the signalling standard used by telephone systems to transmit keypad inputs — the tones you hear when pressing digits on a phone.

When you press a key, your device generates two simultaneous audio frequencies: one from a row and one from a column of the frequency matrix. The combination uniquely identifies each digit. For example, pressing 1 produces 697 Hz + 1209 Hz at the same time.

This system uses 8 audio frequencies in pairs to represent 16 signals: the digits 0–9, letters A–D, and the # and * symbols.

Mark refers to the duration of a DTMF tone, and Space refers to the silence between tones.

In traditional telephony, DTMF tones travel as audio within the phone call itself. Over VoIP and SIP, there are three different methods for transmitting DTMF — each with its own advantages and compatibility requirements.

Telnyx supports all three methods, and you can configure which one your SIP connection uses directly in the Telnyx Portal.

RFC 2833 — also known as RTP Named Telephony Events — transmits DTMF digits as special RTP packets, sent alongside the audio stream on a dedicated payload type (typically PT 101). The tones are separated from the voice audio entirely.

Why use it:

Best for: The majority of Telnyx SIP deployments. If you are unsure which to choose, start with RFC 2833.

Inband DTMF transmits tones directly within the audio stream itself as actual audible frequencies — exactly how DTMF worked on analog phone lines. No special signalling is used; the tones are part of the voice audio.

Why use it:

Important limitation: Inband DTMF is susceptible to distortion from audio compression. Codecs such as G.729 or Opus can degrade or destroy the tones, causing missed or misread digits. Inband is not recommended when compressed codecs are in use.

Best for: Legacy analog adapters (ATAs), older hardware, or where the far end only supports inband tones.

SIP INFO transmits DTMF digits as SIP signalling messages — completely out of the RTP audio stream. The digit is sent through the SIP dialog itself, separate from voice media.

Why use it:

Limitation: Can have timing and synchronisation inconsistencies compared to RFC 2833, and is not supported by all SIP devices.

Best for: Scenarios where the destination device or hosted platform explicitly requires SIP INFO, or where RFC 2833 is not supported.

You can change the DTMF type for any SIP connection in your Telnyx account by following these steps:

If you have any questions or run into issues with your DTMF configuration, the Telnyx support team is available 24/7.