This has been an on-going problem for over 6 months now. The final result with Avaya was to send them the IPO configs from both sides of the network and let them plug them in on their network. Their results were that it worked, so the conclusion was that it is my firewall.
I have heard rumors from other IPO business partners that the IPO is technically not compatible over a NAT'd network (ie. cable modems). My VPN is a true site to site VPN so all ports are open, I don't understand your last statement about the IPO and Firewall using the same ports.?.