DTMF recognition is not supported

Currently, cairo replies to DTMF events with error log output
 ```
53159 [.168.71.1 port 42050] ERROR                     System.err (   218) [] No format has been registered for RTP Payload type 101
 59206 [RTPEventHandler     ] WARN  echforge.cairo.rtp.RTPConsumer (   237) [] Received an RTP PayloadChangeEvent.
Sorry, cannot handle payload change.
```
