Simple definition
A SIP redirect server returns alternative contact addresses instead of forwarding the request.
Purpose and use
A SIP redirect server tells a client alternative destinations for a request instead of forwarding it like a proxy.
How does it work?
It returns a 3xx response with contact information. The client then determines subsequent requests according to its rules.
Example and verification
If a call stops after redirection, check the offered URIs and whether the client can reach them using the correct transport.
Points to watch
Redirection neither establishes the call nor guarantees that the alternative remains available. Client policy may also refuse to follow it.
In short
The server supplies another destination; subsequent routing remains the client's or its infrastructure's responsibility.