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.
Do not run any of the header rewriting for messages. This will
simply relay SCCP friends to the MTP library. Some work on msc_conn.c
is needed to avoid calling most of it.
Holger Hans Peter Freyther