[Users] Proxy to Proxy Communication

Dear Members! I am very new to This VOIP Technology. I have just installed the Openser. I am now able to Do the Communication between the users who are resisterd in the proxy but how do I establish the calls to another proxy.I mean Lets consider I have two Proxies and I want a user of Proxy 1 to coomunicate with a user of Proxy2.How will I edit the configuration files so that I can achieve that. And One more thing.How do I achieve the Nat Traversal in openser.Let us consider I am running my Server in 24.10.65.90(Public and static).and I have a Lan connection In the client side, When the two phones from the same LAN want to talk with each other by using the proxy at 24.10.65.90, Its not possible.I got Disconnected from the internet connection and also when I tried this.If I am calling to a Phone other than this LAN using proxy at 24.10.65.90, I am able to establish.What may be the problem to the previous. I hope I have explained my problems in a user Understandable way. I am expecting a sooner reply. Thanking you In Advance, Shiva