Try putting the Voipo ATA behind your cable modem or your broadband/DSL modem. Many broadband providers do not limit which mac address connects to their modem.

At least this will skip your router and gives your voipo ATA an external IP to determine if it's your router or not.

If you cant call out at all then your broadband provider may have limited the mac address.

If your calls are clean/better then router needs adjustments/config/changes.

If it's the same, then as others mentioned, could be the hops between you and voipo gateway maybe.

Some routers with DMZ aren't really hardware DMZ but software DMZ, so removing the router all together is not always the same as adding a DMZ.

good luck