1. Login to the Zyxel router admin panel. 2. Click on "Expert mode". 3. On the left hand side of the window, click on the Settings button (small gear icon). 4. Under the Network section, click on "NAT". 5. Click on the "Port Forwarding" tab. 6. In the Service Name drop down box, select "VoIP (SIP)". 7. In the "Server IP Address" drop down box, select the IP corresponding to the VoIP device. 8. Click on "Apply".
If it doesn't seem to work, try rebooting the router and your VoIP phone should be functional once it comes back online.