Want/need a crash course on Session Initiation Protocol?
EnterpriseVOIPPlanet.com's four-part Understanding SIP series is the best I've read, hands-down. Technically sophisticated and authorative, but understandable to non-techs.
The series is written by Mark Miller, president of Denver-based consulting engineering form DigiNet Corporation and author of numerous books and white papers about VoIP and networking.
Here's a clickable list to each article in the Understanding SIP series;
Part I- History and Architecture;
Part II- Protocol Capabilities;
Part III- Message Formats, and:
Part IV-Describing The SIP Session.