Ensure the following:
When you log in on VoXalot, Home Page, there is a NAT Symmetric Handling Setting (make sure it is set to yes)....
Under the details for the Voice Service Provider in question, Optimize Audio should be set to NO....
As Martin mentioned above, arrange the codecs as suggested....
Once you've changed this settings in VoXalot, I suggest powercycling your ATA (power off, wait a few min, power back on)
This powercycle may or may not help the time within which the settings take effect (the latest the settings take effect is when your register expiry time is set in your ATA, by default this is 3600 sec, 1 hour, unless you changed it)
Once your ATA is up, test *600, then test placing a call with your provider, if it is not working right away, you may want to give it 1 or 2 hours....
Hopefully it'll work
!!! (As you may realize VoXalot is set up to handle it, but whether your provider is happy with a middleman may or may not be a factor)
Question: Why do you need to only use G729, is it a bandwith requirement?