[Users] Newbie needs some (transparent) proxy help

Hi I've just had a project dumped on me to build a sip proxy to sit in front of our Nortel MCS setup (and ultimately other VoIP gateways). Previous admin was supposed to have done this but he recently been given the boot and hadn't even touched this project from what I can see. And I'm a VoIP newbie which doesn't help much - keen to learn but defo a newbie Setup is fairly straight forward from what I can tell but I seem to be doing something completely wrong so seeking help. New sip proxy (OpenSER) is to sit in front of the MCS (and in time other boxes as I mentioned) and it should proxy traffic to/from the sip servers based on the end users realms/domains (i.e. 1234(a)test1.sip goes to server X and 1234(a)tes2.sip goes to server Y) - I'll need to add an authentication layer here but one step at a time. There's no NAT'ing or anything else going on - everything (clients, sip server & sip proxy) sits on public IP space. All I've been able to get to work so far is registration but can't make or receive calls. I've spend 2 days now trying to figure this out (newbie eh ?) and am under increading pressure from management to get this sorted so am getting desperate. I've googled this to death and tried all the various configs that sounded like they would do the job - but none have. I'm sure someone has already done this and would appreciate if you could share the config with me ? thanks dave