Now we come to the rtpmap lines for the formats included in the “m” lines above to map the payload types and the codecs. We also have a number of the fmtp lines to describe characteristics of the offered codecs. The order in which the formats are written in the media line is important as it defines the priority in which the browsers would like to use them.
This line indicates the browser supports RFC4733, allowing it to send DTMFs within the RTP not as the usual digitized sine waves but as a special payload (in this case with payload 126 in the RTP packet). This DTMF mechanism ensure that DTMFs will be transmitted independently of the audio codec and the signaling protocol.