Well, with the constraints imposed by you, T-Mobile and GV, you have IMO only two choices:
1. Find a way to send calls to your GV number from a different number, which you would whitelist. VoxOx is an easy way to do that.
Or,
2. Get a new number that rings to your OBi, which bypasses all the GV stuff. Callcentric + IPKall is one way to do that. I recommended that specific way, because for $1.50/mo., Callcentric will provide 911 service for your family, as well as a backup in case of a problem with GV. However, for your current problem, you can use a free Callcentric account as part of the solution.