What *I* was trying to explain is simply rephrasing what OBi support said:
Communication between the OBiTALK back-end provisioning system and Google Voice is only necessary during the initial configuration (set-up) of a Google Voice account on your device. During this procedure, the provisioning system, with your explicit permission, creates an OAUTH security token, which is stored on your device. After that's done, there is no further interaction between the OBiTALK servers and Google Voice. Your device is registering directly with the Google Voice service on your Google account, using the token.