There is a command which for some reason I am unable to remember of the top of my head, that allows you to see this on the IOS per ephone I believe...
This is one method, to which was used frequently when I did an ISR 2800 deployment with 30 remote CIPC staff coming in on the system, but I suspect your question though is pertaining more to a SIP trunk and the users experience of that right?
Wireshark is good for this although you really have to wade through a truck load chatter and at times can be confusing.
As soon as I can remember or even find the command I will post it for you to check that one out as well.
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