For troubleshooting purposes, I'd recommend one of the two following tests:
1. Try putting the PAP2 in your router's DMZ--ports 5060-5079 are for the call setup, but not the call audio. Perhaps someone in tech support can tell you the RTP ports and you only need to open them. (You might also try 10000-20000.)
2. Take the router out of the picture. Obviously, with the PAP2T, this is a very short-duration test, and it's a pain. However, if you don't have audio problems when the device does not have to handle NAT translation, you might be able to track down what settings in your router that you should tweak (or if it's another problem altogether).