i finally figured out how to use AT&T Callvantage behind a firewall, I am actually not sure why i've wrestled with this for so long - it's pretty easy:
1. plug your firewall router WAN port into your cable/DSL modem
2. configure the AT&T box for a static internal IP on your network (example 192.168.1.X) and plug the WAN port of the AT&T box into a normal networking port of the switch
3. port forward 2427 with both TCP and UDP to the static internal IP you gave your AT&T box
that's pretty much it. works much better this way than if it is upstream of the router for me.