[Users] mediaproxy and subnet

Hi, I have set up openser on a public ip address running the openser.cfg from the getting started tutorials (http://siprouter.onsip.org/doc/gettingstarted/ch08.html), very slightly modified... and it all runs great with mysql users and locations, mediaproxy for far-end nat traversal. Cool. For my next challenge I would like to find a way to disable the mediaproxy call routing if the 2 phones are on the same private subnet. In essense I think I ought ot be looking at INVITES to see if the first via address is the same public ip, and if so then don't fix_nated_contact and don't use_media_proxy. The reason to do this is that if 2 phones are on same subnet they should be able to see each other's private ip and so there is no reason to send the whole rtp traffic out over their adsl connection to mediaproxy and back to the same adsl connection! I think I need some logic in the following sections: Loose Route Section, in the if method="INVITE" block (to handle reinvites) the route [3] block INVITE MESSAGE HANDLER to set up new call route the route [4] block NAT TRAVERSAL SECTION for routing established calls the onreply_route[1] block .... not sure I understand this block yet... Am I going in the right direction? Has anyone got some working snippets for the openser.cfg or some references to a useful page on the web? Thanks.