In telco tracing has always been very important. It helps us to understand the flows, the content and – sure – to find bugs. SS7 used to be much more strict and we didn’t need to go in the detail traces that often. The flow and a few fields was usually enough. With SIP and SDP it is a different story. SIP is a very flexible protocol. That means powerful but also complex and sometimes quite confusing.
The registration flow is very important – in practice if something goes wrong, it is most probably registration. Once is a subscriber correctly registered, the other flows (call, sms, fileshare) usually works fine.
There are many documents describing the message flow and important values. For VoLTE I’d recommend to go through VoLTE Service Description and Implementation Guide or A Definitive Guide to Successful Deployments. Would you have any doubts about a particular header please refer to http://www.iana.org/assignments/sip-parameters/sip-parameters.xhtml. Now let’s go through the particular messages.