That's it!
I need to watch the headers between VoxaLot and a SIP provider.
That's why local logging won't help much.
I found a
tool, which seems to be useful, but there will be too much efforts with configuring the port forwarding to get SIP traffic routed from VoxaLot to my PC and further to a SIP provider.
Another way is to setup an account at
mySIPswitch and enable tracing there. Then forward all calls thru mySIPswitch, which will result tracing.