We're on XNET's openvfx, which has been great.
We used a Linksys SPA2102 for about a year, and when we got a second line there was often a stutter. Swapping setttings cables, phones- nothing made any difference, it was whatever was plugged into the 2nd port that stuttered, and in the end it seems it was a hardware fault, as an upgrade to a Cisco SPA112 fixed the problem. The point: that occasionally ATAs do seem to die, so don't overly stress yourself or abuse xnet if there's a problem: might be the hardware.
So, voice quality now fixed with the SPA112. However, a firmware upgrade to v1.2.1 (out about July 2012) the unit would freeze on a regular basis (every week or two) requiring a power off/on to bring it back to life.
Eventually Cisco tech support said that the v1.2.1 upgrade requires you to also reset to factory defaults, and then re-enter all your settings (don't dump the config and re-upload it).
Anyway, that fixed the problem, so hope this saves someone else faffing around.