SIP, RTP, SDP, RTCP, OH MY!
To troubleshoot VoIP you need to understand at least a little bit of SIP and related protocols. That's the sad truth. You will get nothing from packet-capture and analysis if you don't understand the basics of the protocols.
You can certainly send the file containing the captured packets to someone who's more SIP conversant. If you want to take that option, read the following section about the tools you can use to generate a pcap file with all the relevant info.
The other option is to learn it yourself before problems occur, using sipgrep and sngrep (they make for a colorful, easy and complete toolset) to visualize protocol packets, and one of the many books/tutorials out there as reference. Do test calls and watch what happens. You'll see that there are regularities and meanings, and after a while you'll know most patterns and what to look for. You'll be ready when the time comes.
