Actually this problem seems to relate to some issue with VoXaLot / SipBroker.
We tried creating some new accounts with both.
We found that without changing anything on the Asterisks machines that some accounts would work ok, yet others would drop out the call after a few seconds or the DTMF would not work.
All very strange but basically nothing can be fixed at the outside Asterisk end.
The accounts that fail need to be looked at the VoXaLot / SipBroker end.
.-.-.
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