Also called a SIP server or SIP proxy server, in telecommunications it is one of the main components of an IP PBX that is used by SIP to perform many of the call set-up functions. As described in RFC 3621 - SIP: Session Initiation Protocol SIP makes use of elements, called proxy servers (SIP proxy) "to route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users".
Within the SIP network the SIP proxy actually manages the setup of calls between SIP devices including the controlling of call routing and it also performs necessary functions such as registration, authorization, network access control and in some cases it also handles network security.
See Session Initiation Protocol (SIP).
See also "Understanding SIP Part V: SIP Signaling" on Enterprise VoIP Planet.
Also see "VoIP and the Small Business" in the "Did You Know..." section of Webopedia.
Also see "Internet Telephony" in the Quick Reference section of Webopedia.