DTMF-Source may have the following values:
   APP : injected by application (send_dtmf, etc)
   ENDPOINT : detected by endpoint signaling (like SIP INFO)
   INBAND_AUDIO : detected by start_dtmf, spandsp_start_dtmf, etc
   RTP : detected as 2833/4733 telephone event
   UNKNOWN : unknown source

One possible use of this header is to determine telephone events
are being received, and if so, disable inband detection.