(Session Initiation Protocol proxy) A proxy server that allows SIP-based telephony packets to traverse the network's firewall. The SIP proxy takes over call control and provides address translation in order to direct calls to the appropriate phones within the network. Deployed in companies with many SIP phones, a SIP proxy offers a very comprehensive solution for enterprises. For example, it supports authentication, encryption, telecommuters and can even serve as backup for the PBX.Stateless vs. StatefulA stateless SIP proxy establishes the connection and gets out of the way. A stateful SIP proxy keeps track of the call and supports more sophisticated functions such as call routing and caller filtering. See SIP ALG, SIP and proxy server.