It should simply work. I just now tested, adding another one of my Google Voice numbers to one of my OBis, and it works just as expected.
Power off your OBi, your router, and your cable or DSL modem. Power on the modem, wait for it to boot up, then power on the router, wait for it to boot up. On your computer's web browser, on the OBiTALK web portal, delete the OBi 200 completely off of your account. Wait a few minutes. Unplug all three cables from the OBi (power, phone and Ethernet). Get and un-bend a paper clip. Turn the OBi upside down. Gently insert the paper clip into the reset button's hole. It is a slightly raised, round bump with a hole in it. Feel for it to click the switch. Continue to hold it down, while inserting only the power plug into the OBi. This takes some manual dexterity; you may need a third hand to help you. Continue holding down the switch for at least 10 seconds. Release the switch. Wait several minutes while it factory-resets.
Log into your Google Voice account on the web. If you do not already have a forwarding/linked phone number added, add and verify one now. It can be any 48-state US phone number. After you do that, call your GV number from some OTHER phone number (NOT the linked number). It should ring through to your forwarding phone, and you should be able to answer the call. If that is successful, then...
Now, go back to the OBiTALK web portal, and follow the **5 procedure to re-add the OBi to your dashboard. Set up Google Voice on SP1 again. What happens?
If it doesn't work, then you have some sort of problem with your internet service.