What is DTMF RFC 2833 used for?

• A. In-band signaling of DTMF tones
• B. Signaling DTMF via SIP INFO messages
• C. DTMF tones are not supported
• D. Out-of-band transport of DTMF tones as RTP events

Answer: (D)

DTMF RFC 2833 is about carrying DTMF digits over the RTP stream as separate events, i.e., out-of-band signaling. Instead of embedding the tones in the audio waveform, the sender transmits DTMF as RTP payloads that encode an event for the digit, with a duration and an end indicator. This approach avoids problems that come from audio compression, VAD, or transcoding that can distort in-band tones, making DTMF detection more reliable across diverse networks and codecs.

The digits map to specific DTMF event numbers (0–9, star, pound, and A–D), and the receiver interprets those RTP events to produce the corresponding DTMF tones on the user side. While other methods exist, such as signaling DTMF via SIP INFO, or relying on in-band tones within the audio, RFC 2833 specifically designates the out-of-band RTP event transportation for DTMF.