SIP or "Session Initiation Protocol" is a means of making your calls work.

It is the new standard being used in the 21st Century Networks (21CN) and it is core to it's function, any next generation telephony system should have this standard in order to work.

Brief Overview of it's functions:

• Open standards based system for greater interoperability between systems and suppliers
• Set up calls and sessions for Voice, Video, Messaging Services including Instant Messaging and conferencing all using just one single identity (SIP URI or otherwise known as your email address)
• It's an extensible architecture designed to support the rapid deployment of new services across a multitude of infrastructures including wired and wireless.
• Use it to connect both phones to the system and also the system to other systems or telecommunications operators (SIP trunks).