(I called Digium telephone support, but they advised me to post something here

I would like to schedule a specific training session to learn how to interpret/analyze the SIP debug data or the packet capture with Wireshark/...?
Is it possible to receive a specific training like this?
I'm running multiple Asterisk servers and I noticed that random extensions are going unreachable on Asterisk 13. Once I downgraded to Asterisk version 11, these unreachables were gone.
With a better understanding of the SIP debugs & captures, I hope I can troubleshoot what is going wrong.
Thanks a lot for your help,
Frederik