The RTP payload type of the negotiated DTMF-relay.
RTP payload specifies the data transported by RTP in a
packet, for example audio samples or compressed video
data. A value of -1 means that no codec has been
negotiated.
RFC 3551 specifies the payload type number to be used
with different codecs. The following are some of the
standard payload types applicable to SIP calls :
PT Encoding Name
0 PCMU(G711ulaw)
3 GSM
4 G723
8 PCMA(G711alaw)
9 G722
15 G728
18 G729
96--127 dynamic
For other codecs, dynamic payload types in the range
96--127 are negotiated during call setup.
Also note that Cisco has preassigned certain payload
types in this dynamic range for certain encodings:
PT Function
96 fax
97 fax-ack
100 NSE
101 NTE
121 DTMF-relay
122 Fax-relay
123 CAS
125 ClearChan