UA1 ----------------------> Proxy 1(not openser)---------------------- Proxy 2(openser)<----------------------UA2

UA1 is registered to proxy 1 and UA 2 to openser. I wish to make a call from UA 1 to UA 2. Proxy 1 and proxy 2 are in the LAN but do not know of each other. Is there a way we can configure the openser to send/receive the invites to proxy 1? I think the calls will be handled on domain basis, so openser will treat all the UAs registered to it as local and allow them to call each other. But how to make it allow inter domain calls?