I'm afraid you aren't clear on how this works, and it gets pretty tedious explaining things from the ground up.
Google Voice, among many other things, is a SIP VoIP telephone service provider. Callcentric is also a SIP VoIP service provider. Technically, one can simply use Google Voice on an OBiTALK device to make and receive telephone calls. However, it lacks two features: E911 calling, and Caller ID Name (CNAM). To gain those two features, you can also sign up for Callcentric. You then have
two different service providers provisioned (set up) on your OBiTALK device.
Technically, you can also not use Google Voice at all, if your calling volume (total number of minutes on calls) doesn't exceed your willingness to pay for Callcentric's telephone service.
However, the beauty of Google Voice is that it is also an integrated call forwarding, SMS/MMS and voicemail service, that works on land line PSTN phone numbers, mobile phone numbers, and OBiTALK devices.
Google Voice can forward inbound calls to any combination of:
- up to six 48-state US 10-digit PSTN phone numbers (mobile, land line and/or VoIP)
- multiple Android or iOS devices running the Google Voice app (which, like an OBi, is a SIP client)
- multiple Firefox or Google Chrome Browsers on the Google Voice website
- as many OBiTALK devices as you wish