I have been with VOIPo since before the beginning and this is what I have that gives me excellent results.

Since I was around before, I have the privilege of being able to use any of my collection of ATAs and program them myself. Since we're talking about phone service and since I haven't had a land line for 7 years and I have to please my wife, I long ago decided to find the best settings and leave them alone.

I don't ever use STUN and I wouldn't ever put anything in the DMZ. In my router, I have SPI and ALG off. I have a third party firewall as the router firewall causes me problems.
I have the ATA set with a static IP and the router ports 5004-65000 forwarded to that static IP.

My ASUS router has DD-WRT and I don't use any QOS, since for me, it causes more problems than it is worth and my DSL speed is 6/786.

The above settings offer me a connection that rivals POTS in both connectivity and audio quality.

I wish I could take credit for all this, but the credit must go to Brandon\VOIPo who spent many hours over the years tutoring me and ultimately convincing me the right path to take.
My basic nature is to effect change for the sake of change, but as I mentioned, this is phone service and not really the place to fool around and then blame the provider when it doesn't work.

To be fair, everybody has a different setup and talks and listens on different circuits, so there will certainly be differences. However, talk to VOIPo...they do know what they are doing.