Session Initiation Protocol (SIP) is a signalling protocol used for establishing sessions (usually but not limited to a phone connection) in an IP network.

It seems that the world has finally embraced SIP (Session Initiation Protocol) as the defacto method for phone communications. We see it everywhere. I don't think there's a legacy PBX manufacturer that hasn't either announced a SIP version of their PBX or at least released, or planning to release, one or more plug in boards that will allow the use of SIP devices.

SIP was originally defined in 1999, by the Internet Engineering Task Force (IETF) in RFC2543 and was designed for initiating, terminating, and modifying communications primarily between people or software agents. RFC2543 has been replaced with RFC3261.

SIP is perfect for intelligent control of phone calls and messaging and represents a huge improvement over the complex AIN messaging that's been used within the Telephone companies for decades.

SIP has removed that barrier many companies had requiring them to acquire and learn complicated binary communications protocols. Now, we have a large number of companies all working, not towards telphony, but towards communications.

However,  like many new technologies, SIP is being touted as the answer to all problems. This is, of course, not true. Even though there are a large number of extensions with more coming all the time, SIP is not the answer to everything.

SIP is, however, a great boon to the communications industry and has helped usher in a new era of features, competition and choices for consumers.

Another great aspect of SIP is that  a PBX can be built far less expensively than that of all the legacy PBX's. This is mainly because proprietary protocols no longer need be created and debuged and the manufacturing cost of printed Circuit board for legacy PBX's has been replaced with software when using SIP.

Compared to the legacy PBX, a SIP based PBX, is much easier to add and change features and is a more reliable device simply because of the far fewer physical components than in a legacy PBX. However, because SIP and VoIP are relatively new, there are fewer people who understand the intricacies and nuances of the technology.