The Session Initiation Protocol (SIP)

The Session Initiation Protocol (SIP) is considered by many to be a powerful
alternative to H.323. It is considered to be a more flexible solution, simpler
than H.323, easier to implement, better suited to the support of
intelligent user devices, and better suited to the implementation of advanced features. Although H.323 may still have a larger installed base
than SIP, most people in the VoIP community believe that the future of
VoIP revolves around SIP. In fact, 3GPP has endorsed SIP as the session
management protocol of choice for 3GPP Release 5, albeit with some
enhancements.
Like H.323, SIP is simply a signaling protocol and does not carry the
voice packets itself. Rather, it makes use of the services of RTP for the
transport of the voice packets (the media stream).