Hi, I've implemented a system to control international destinations (allow/deny) using "PDT" module (based on prefix).
In OpenSer 1.4 there will be a new module "userblacklist" which can handle this issue in a more ellegant way, but for OpenSer 1.3.X we can use "pdt" module (with a little patch to pdt.c) to implement it.
I've submitted the patch and the how-to:
https://sourceforge.net/tracker/index.php?func=detail&aid=1965211&gr...
Hope it could be useful. Regards.