Currently, we user DTMF to inform the client when the call session is in echo test and when entering the voice conference.
 Unfortunately, sometimes when FS sends the DTMF, FS crashes.

 Monitor the progress of the call session using ESL events and propagate to the client.

 The client would be informed of these call states: CALL_STARTED, IN_ECHO_TEST, IN_CONFERENCE, CALL_ENDED.

 - When waiting for a response from FS after sending an ESL command and the ESL connection disconnects,
   the sending thread will be blocked as the trigger for it to unblock is the response from FS which never
   comes.

   Add a 30 second timesout waiting for response and give up to go send a new FS ESL command.

   listen only callers. If not, the globak audio stream is destroyed and not re-recreated
   resulting in no audio for listen only.