There are three common ways of transferring DTMF in SIP Or VoIP

1) SIP INFO Messages
This is the out of band method (out of band means that the DTMF messages are not carried within the media stream i.e. the RTP and carried separately
Its format is very simple, just in the body of INFO message the pressed DTMF digit is mentioned along with a duration for the DTMF signal

2) In Band DTMF transfer

As the name says, in this method the DTMF tones are carried in the media stream or RTP codec. This is a very crude method, DTMF is carried just a normal voice and we have no control over the DTMF , we can’t see it or decode it using wireshark or any other protocol analyzer
The optimized voice codecs like G729, GSM and G723 which used limited bandwidth face lot of problem in transferring in band DTMF
Due to limited bandwidth DTMF signals do not properly get transferred and receiving party is not able to decode them
To overcome this problem the 3rd method was devised

3) DTMF in RFC 2833

Via RFC2833 basically defined a new payload which has been created specially for carrying DTMF tones
So that DTMF is not dependent on the codec which has been made for voice but has its own optimized codec
Support for RFc 2833 is shown by the SDP rtpmap telephone-event


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