Message diagrams are a great way of visualizing SIP calls.
When troubleshooting SIP issues, you can use Wireshark to automatically generate such a diagram by selecting Telephony, VOIP Calls, Flow:
While such a diagram shows you what is going on, it might be hard to spot the problem if you’re not sure what it is supposed to look like.
Luckily the following site provides example diagrams for lots of common situations (hold/transfer/…):
For an explanation of various SIP aspects (e.g. timers, headers, tags,…) you can also take a look at the articles on the following site: