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Theoretically, SIP servers are :
1-Proxy server that is responsible of security, routing and services.
2-Registrar server mainly responsible of authentication.
3-Redirection server that is used to redirect some requests to other SIP servers it can be configured on request basis or as a global redirection server.

Those servers can read SIP messages. (i.e. SER is a open source tool that is a SIP proxy server).

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Posted by george99