Simple definition
A SIP proxy routes signaling requests between endpoints according to network policy.
Purpose and use
A SIP proxy forwards signaling to route requests toward the appropriate next server or endpoint.
How does it work?
It examines the destination and routing policy, adds required path information and can fork a request toward multiple contacts.
Example and verification
For misrouted calls, compare the target URI, registered contacts and routing headers at each proxy hop.
Points to watch
A SIP proxy does not necessarily relay audio. Its role differs from a B2BUA that terminates and creates signaling relationships.
In short
Proxy diagnosis primarily concerns signaling; media paths and protection require separate analysis.