Hi Tom,
As far as I know, there isn't a way to configure a policy there. In real life, you don't assign a policy to the mediation server gateway anyway... it only really needs a location profile.
I think this is more a goof in the route helper... If you are able to have the route check out with the user/manual you probably have everything working right.
OCS will send an error back to the client if it doesn't have a route and you'll get fast busy; since your calls are ringing I think that you've set up OCS properly. It seems more like an issue w/ the audiocodes.
Check the event viewer in the MP-118. It will show you if the call is getting to the MP-118 or not. If it is getting to the gateway, you may need to tweak your config on the audio codes.
Matt