Debugging re-invite issue

I have found a “bug” visible by both Asterisk 13.5 and 13.6. The question is how to debug it and find the root cause. My provider has various routes. When I connect to 3 out of the 20 routes a re-invite from my sever to allow the phone and the provider to directly communicate causes people to not be able to hear me. All looks OK in logs from what I see.

The question – what debug levels or where can I add logging to find out what is causing the issue. I need to find out whether this is a bad implementation of the re-invite by the provider, some from of error that Asterisk isn’t handling or some form of firewall issue that is external to my network. When this happens audio works for about a second and then just abruptly stops when the re-invite happens.