Websites for better understanding of SIP protocol.
I researched on SIP trunking VOIP phones previous two years ago, I developed a draft VOIP Technical Review and Risk Analysis for my organization. The SIP is nowadays captivating innovative trend where most of the organizations utilizing Cloud SaaS is reducing risk. However, I gained the control knowledge from ISACA.org where you will get better understanding about the implementation of this technology. Stab here if you need deeper understanding of SIP: http://www.sipforum.org/content/view/19/72/ or techtarget.com where search for SIP gives VoIP technology a boost or you may get more insight from https://www.owasp.org.
@Shantha_mh - good post. A thumbs up from me, and someone else as well. SIP - Session Initiated Protocol, used for VOIP (voice over internet phone) is very important these days. Good reference links are important to understand what this stuff does.
YOu should try siptutorialand sipforum !!