I spent some time on this earlier today.
All my analyze with WireShark and VoIP are like that:
Menu Telephony > VoIP Calls
I select the call and open "Flow Sequence" and do screenshot.
I do the same with other one and compare both screenshot's - the difference like, differ ports, no RTP streems.... give me information about communication.
Menu Telephony > VoIP Calls > Play Streams give me feedback about problems like quality etc.
I grab/sniff cap file from last branch router who have end user and from DC router who have a VoIP PBX - with that I can play recorded sound who is near PBX and Phone and listen problems at both directions.
Maybe this help you.
Remember that some PBX need have got DNAT and ros in beta 5|6 vs 7|8 maybe not give you Public IP and that's why this not work. Then this will be outside wireshark with voip filter.