Lets get to the simplest config:
You have a provider (adsl,cable or phone line) that carries your internet to your location.
That conection is the one that have the static I.P. address.
From the cable modem , or ADSL modem,should be a ethernet cable to the WAN port of the router.
The router shoul have the static I.P. address setup on the WAN.
On the inside, or LAN, you will be using DHCP or manually asigned I.P. addressses for your devices.
Somwhere on the setup of the router should be a DMZ tab or setting.
It normally ask for the internal I.P. of the device that you need to set as part of the DMZ zone.
Some other routers have a specific port that is the one that your could set as DMZ.
I do not know where you are at, but Linksys routers are not too expensive, and guaranteed to work on most cases.
If you still need help, feel free to contact me at mike3799@yahoo.
I could send you the file with the ATA setup for H323 and the sample of the linksys and DMZ zone.
Best regards.