Proxy or Proxy Server/Policy and Routing Server

Proxy or Proxy Server/Policy and Routing Server in the context of SIP is a proxy server, an intermediary entity that acts as both a server and a client for the purpose of making requests on behalf of other clients. A proxy server primarily plays the role of routing, which means its job is to ensure that a request is sent to another entity “closer” to the targeted user. Proxies are also useful for enforcing policy (for example, making sure a user is allowed to make a call). A proxy interprets, and, if necessary, rewrites specific parts of a request message before forwarding it. It can be a policy/routing element in other protocols.

RFC 3261, SIP: Session Initiation Protocol