When looking for SIP traffic in Wireshark, you can start with the Display Filter
sip. Here are a couple more specific filters.
To find a specific type of SIP packet:
Sometimes you need to match registration traffic on the server and client (two Wireshark sessions). To see matching traffic, in a SIP Message Header, find a Call-ID on one side, then use a display filter like this on both the server side and client side:
It may be helpful to combine several filters:
ip.addr = 126.96.36.199 and sip.CSeq.method==REGISTER and sip.from.user==120