Once a connection is negotiated, SIP gets out of the way, which makes it scalable. Servers can be used to locate and connect all the endpoints involved in a session, including those that support IM. SIP-enabled voicemail and IVR systems can be added to the VoIP PBX function easily, as long as there's IP access.
SIP is also ideally suited for presence applications and can integrate with Web applications, easing CTI (computer-telephony integration) and opening up more options for features like "click-to-talk."
Development and support are simplified by having one protocol and architecture. Any application that requires instantaneous communication is probably already supported by SIP or will be. The beauty of this is that one simple infrastructure, consisting of a SIP Proxy Server and a SIP Registrar Server, can direct requests and track location information. Additional applications can be added with little disruption.
Sure, it's possible to accomplish all this using proprietary systems. However, that route is not necessarily easy and may lock you into a single vendor. Not surprisingly, some vendors with big investments in proprietary alternatives argue that SIP is still a work in progress. But after testing VoIP phones, we say SIP brings to the table solid functionality and interoperability. (For the nuts and bolts of how SIP works, see "It's Time To Take a Look at SIP").