There isn't really much to do here in the 5000 if another phone is working already you have most of the battle won... You basically just have to tell the system that the phone is outside the firewall.
Go to 5000 CP->System->Devices and Feature Codes->Phones->XXXX->IP Settings and change the NAT Address Type to NAT, and optionally change the Call Configuration to the one for remote phones (typically 2)
After that, throw the phone in Teleworker Mode and enter your public IP address and your done.