I always advise people to not waste their time trying to manually stare at all the hundreds of variables, looking for the problem(s). It's time consuming, iterative because you may find one thing and miss another, and it's just not worth it.
It only takes about 5-10 minutes tops to follow my advise and simply start over. Doing this will automatically download the correct customization for everything that needs to be changed from default.
I have set up a very large number of OBiTALK ATAs and IP phones, so I'm fast, but anyone who can follow instructions should be able to do it easily.
Now, that said, if you insist, I've captured a screenshot of how Google Voice should be configured on the Voice Services-->SPx Service page.