From Yate Documentation
The call.progress message is emitted by a channel-creating module (such as h323chan and ysipchan).
The message has been designed for asynchronous processing (broadcast).
- yes or no -- currently, the code sets this to yes unconditionally
- the remote IP address of the RTP channel.
- the remote port of the RTP channel
- a comma-separated list of supported formats (codecs) advertised by the remote end of the RTP channel.