[Users] SIP Firewall

Hello, I'm Sergio... Can anybody tell me if OpenSer could be used in the following way: ANY SIP REQUEST != REGISTER (to port 5060) ----------------------------> OpenSER --------> SOME PROXY_SIP (To port 5060) and the REGISTERs Request, redirected to another PROXY SIP and to another port... REGISTER REQUEST (to port 5060) --------------------------------> OPENSER------> REGISTRAR PROXY (to port 5062) I need to use it as some kind of Firewall SIP, REGISTER redirected to some IP and to some port, and the other requests redirected to another IP and port diferent. If this could be done, How? Where should I start reading? Thanks any help, Sergio