Unfortunately for you, Polycom doesn't seem to presently support Google Voice on the OBi508vs--and hasn't done it for many months.
I don't know why on Amazon, Polycom still markets the device as Google Voice compatible.
For now, you are out of luck. Is there legal recourse? I think so--based on the continued marketing as being usable with GV. Is it worth your time and expense? For me, no. Maybe for you. At least try a small claims suit if you live in CA. Let us know but leaving poor reviews on Amazon doesn't seem to be helping people with the 508s.