Because of the issue parsing the MGCP request a '\0' was added to
the queue. This lead to the dtmf scheduler believing that a play
was in progress while the playing code didn't play anything. This
lead to the queue to be stuck and nothing being played at all.
Return the number of tones that should be played and stop using
strlen on the tones.
Create a simple queue for pending DTMF tones, play them using the
MTN API, and then send the next tones once the playback is complete.
The callback and scheduling is done from the same context so no locking
needs to be done.
Holger Hans Peter Freyther