Does SIP carry DTMF?
There are at least two options for carrying DTMF and similar signals in a VoIP network using SIP. First, DTMF can be transported as an RTP payload (RFC 2833). This has the advantage that it provides accurate timing and alignment with the speech RTP packets. Also, media gateways are the most likely to detect and generate tones, so that making it part of the media stream is appropriate. However, under some circumstances, it may be necessary for signaling entities to know about DTMF signals. Currently, there is no standardized solution within SIP, but it has been proposed to carry DTMF information in SIP INFO messages, either encoded as simple text or using the RFC 2833 format. The latter is more complex, but offers duration and timing information.
