[Users] SIP Proxy configuration

Hello I need help in configuration. Lets say I already have a SIP Server (anyone) and I use it to handle VoIP in my company. Now I want to use OpenSER as a SIP proxy between the UAs and the SIP Server. I want all SIP messages to pass via OpenSER and the OpenSER shall be able to reject an INVITE message in both direction (incoming call and outgoing call). That's all. All the rest shall be handled by the SIP Server (forking, call forwarding, PSTN Gateway connectivity, etc...). Actually I just need the OpenSER receiving the requests, adding itself on route and forwarding to the SIP Server for outgoing requests and to the UA for incoming requests. Is there an example of config file that can do that? Thanks. Regards, Michel.