I don't know how to check this on a 508 or on Google Voice, but I'll pass this along in case it's a clue...
My Obi202 has GV on SP1,
voip.ms on SP2 and SP3, and Callcentric on SP4. After I got everything put back together, I noticed that I could call SP3 and ring the phone, but then answering that phone would terminate the call (in the case of my setup, it somehow went straight to
voip.ms voicemail. And outgoing didn't work too well, either.
The problem turned out to be the codec. While G729 used to work fine, I had to change the setting to include G711u. This is a change that was on the
voip.ms website, not on the ATA itself (and consequently not on Obi Expert either), and hadn't been touched in at least 2 years. That is, I had set it to G729 when I set up that subaccount, it had always worked for over 2 years and then it didn't anymore with the new firmware.
So, some change in the current Obi2nn firmware caused this failure, at least with my SP3 on my 202. Whether this change is duplicated on the 508 is another story. I'm not sure this is a setting on Google Voice anywhere. I've never seen it. But maybe it's a selection on the 508 itself? Or, if you're using a non-GV SIP carrier, check with them. Make sure G711U is enabled.
So, that's my advice: Rule out that it has anything to do with the codec.