SIP FAQ : Relationship to Other Protocols :
How does SIP carry DTMF (touch tones)?
RTP can be used to carry DTMF, as described in RFC 2833. RFC 2833 uses "forward error correction", retransmitting DTMF digits periodically. Thus, unless there are extremely long bursts of packet errors, digits are transmitted reliably. Retransmission by SIP, either at the application layer or via TCP, is based on exponential back-off, with delays of a few seconds after several consecutive losses. If a human generates the touch tone commands, it is possible that such long retransmission delays will cause the user to press the button again, resulting in duplicate digits.
DTMF over RTP is also required to synchronize audio and touch tones at VoIP-to-PSTN gateways.
Gateways that are only interested in detecting tones do not need to buffer audio and can simply forward the audio packets while doing playout buffering and DTMF detection locally.
A number of proposals exist for carrying DTMF in SIP INFO messages, but the working group has not decided which of the approaches, if any, to pursue.
Why use SIP-T as opposed to tunneling SS7 using SCTP?