Not sure how the settings are layed out on a D-604 since I have never used one but they should be in there. With Linksys it's under "Advanced" and "Port Forwarding"
Here are the ports for Game Voice, found these on the MS page....
http://support.microsoft.com/default.aspx?scid=KB;EN-US;Q273874&
Open the following ports to use the Game Voice software through a firewall, proxy, or NAT:
Initial TCP Port 47624 (inbound and outbound).
Subsequent TCP Ports 2300-2400 (all ports in this series, inbound and outbound).
Subsequent UDP Ports 2300-2400 (all ports in this series, inbound and outbound).