Calls not working with Windows

I know, this is really a Windows question but hopefully someone has some experience and can help us out. We downloaded the distro and set it up as a test. It works fine when connected directly to the outside world. However, when it is behind our Windows 2008 box it doesn’t. We have inbound and outbound ports 5060-5063 and 10000-20000 open in Windows firewall. I should specify that you can call out, but neither party can hear the other. Sounds like it may be a NAT or port issue, but we can’t see where. On this Windows box DNS and DHCP are enabled (as many other things). On the NIC which faces the outside world NAT is enabled and port 5060-5063 are forwarded to the static ip of the PBX on the internal network in Windows. However, a softphone connecting directly to the SIP provider, also behind the Windows box, works fine.

Windows security server has never seemed to work right with SIP. The softphone may be utilizing STUN or TURN.