Didn't try by myself... The simple solution would be not using Natting just get a second real IP from the provider or coordinate with your room-mate to split the time to share the line...Did you try other net-phone applications like Yahoo, ICQ and hope they support NATTING...
Now, on the router, when you set up NAT to overload your public IP by your inside real IP's or using DHCP, by default, you are blocking the access coming from outside. So dont forget to map the ports you need to open up to allow access from outside, for example, netmeeting stuff.... so you need to find out what the ports are you need to open first...
Hope it helps.