[Users] Routing

We have a configuration where OpenSER sit's between Polycom phones and Asterisk. Off-net/PSTN calls are also sent from Asterisk to OpenSER to be routed to a Mediacodes Gateway. I'm having a hell of a time with the routing in general! What is the best way to route calls? Should I be looking at the URI? The From:? The To:? The method? Also, the convention seems to be that the URI contains where the message is going TO rather than where it came FROM. However, routing is much easier if the URI has where it came from. For example, messages from the phones have a URI of: uac.voip.com and messages from Asterisk have the IP adress in the URI, as Asterisk doesn't seem to let you set the URI. Calls from the Mediacodes have a URI of: pstn.voip.com Is adding sub-domains like 'uac' and 'pstn' a good way to route? I've looked at all the examples I could find but unfortunately they're ridiculously simple. Every time I look at openser.cfg I get VERY STRESSED OUT! Thanks.