If you've got the ATA ahead of your router (which you do), you won't have to assign a fixed IP or port-forward to the ATA. It just runs silently between your modem and the router. The router thinks the ATA is your Internet connection.

However, I ran into problems with this setup because I port-forward to devices behind the router. That means I have to port-forward through the ATA first -- which I can't do without getting the ATA's password and then altering the VOIPo default settings. Plus I use dynamic DNS from my router, which requires my router to know it's WAN address, but it thinks it's WAN address is the address the ATA assigned it. Most home users don't have these issues, so keeping the ATA ahead of the router is perfectly acceptable for them -- and probably for you.

If you do find yourself in such a position, though, my fix was just to get a newer router. I ended up with a Belkin F7D2301. It doesn't let me do everything I'd like to do, but it was relatively cheap and the ATA works fine behind it without any fixed IPs or port-forwarding.