The brief answer: The main advantage of SIP is its full integration with other Internet protocols and functions, such as email, web and instant messaging. For example, it is very easy to "forward" calls to web pages or email; web pages can be included in responses to call attempts. SIP is also codec-neutral and has been used to set up anything from audio and video to Doom distributed games and screen sharing. There a number of open-standard programming interfaces, SIP servlets, sip-cgi and CPL, that are particularly suited to SIP-based devices. These programming interfaces make creating SIP-based services very similar to writing web scripts or web pages.
It also supports a number of services, such as ACD and follow-me, that are much harder to implement in H.323.
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