DTMF (Dual-Tone Multi-Frequency)
DTMF is the tone signaling produced by pressing keys on a phone keypad. In VoIP it can be sent as audio, as RTP events or inside SIP messages, and the methods must match.
Published 8 November 2026 · 1 min read · By SpaGreen Creative
Short answer
DTMF (dual-tone multi-frequency) is the keypad signaling method that represents each key as a pair of audio tones, used in VoIP to navigate menus, enter PINs and dial feature codes.
DTMF stands for dual-tone multi-frequency. Each key on a telephone keypad is represented by two simultaneous tones, one from a low group and one from a high group, as specified by the ITU-T in Q.23. The tones were designed to pass through the analogue telephone network and are still what voicemail systems, phone menus and conference bridges listen for.
In VoIP there are three ways to carry a key press. In-band DTMF plays the actual tones inside the audio stream. This works with G.711 but compressed codecs can distort the tones beyond recognition. Out-of-band DTMF over RTP, defined in RFC 4733 and commonly still called RFC 2833, sends a special RTP packet describing the digit, its volume and duration. SIP INFO carries the digit inside a SIP message instead of the media path.
Both ends of a call must use the same method. When they do not, keys appear to do nothing, or a single press registers as several. PBX documentation usually recommends RFC 2833 as the default for SIP devices.
SIPdia sends DTMF as RFC 2833 RTP events, which matches the common Asterisk setting dtmfmode=rfc2833.
Sources and further reading
This article is published by SpaGreen Creative, the maker of SIPdia. Product names mentioned belong to their owners. Where it describes other software, treat the steps as general guidance and check the official documentation linked above.
