I am gonna suggest you give this a try as a test.

http://www.pfsense.com/

I assume Comcast will give you full bridge mode on that modem? I know their pretty pushy about use of the gateway modem on their commercial accounts...

I use pfSense at several locations. One with three Voipo numbers, one with a Voipo, and two Vonage numbers, and a Verizon Femtocell. Other locations have no phones.

The only thing I have done is to make a firewall rule for the voipo server I use. Ill share my firewall rules with you if you need.

I use two IP's here at my house and have multiple ports forwarded for various company server activity.

My ISP provides me one address via DHCP and one static.

On my WAN port of the router I use DHCP and then have created a VIP port for the static on the same WAN port for the server.

Like a said- just a suggestion. But you might never look back...

Forum there is full of great info on various setups...