If you want to know more about how SIP works, you might want to look at the freshly updated SIP Tutorial on the Iptel/SIP Express Router site. It even has a section at the end describing IMS and how SIP gets extended to meet the needs of NGNs. The tutorial is created by two SIP pioneers, Jiri Kuthan and Dorgham Sisalem, both of whom I had the pleasure to meet last year and interview for the Blue Box VoIP Security Podcast. It's a very long tutorial, but it's well worth putting aside a couple of hours if you need to know the technical details of SIP.