It seems you also use Tomato, here's a few suggestions from personal experience:
1. Assign a static internal IP to your ATA device based on the MAC address
2. Set up port forwarding to the IP address setup above (dig around your ATA to find the RTP ports it uses)
3. Enable UPnP
4. Disable DMZ(I am skeptic about QOS settings on Tomato as well but leave it be for now...)
5. Use stun.xten.com in your ATA's stun settings
