Here's the thing: you had a very old product, that no longer supported Google Voice. A bunch of unauthorized hacks were made to keep it working. That era is over.
You now have the currently-supported product. It simply works, with no special settings or workarounds.
By design, Google Voice gives you an inbound telephone number. You then "link" or add various forwarding destinations to that GV number, via the Google Voice settings page,
https://voice.google.com/settings.
The forwarding destinations can be up to six different mobie or land line phone numbers, plus Google Hangouts, plus the Google Voice web client if you are enrolled in the GV VoIP calling beta, and as many OBiTALK devices as you wish.
So, an OBiTALK device is simply another forwarding destination. All you need to do is to follow the instructions to set it up.
This has been covered many times in this forum. Very briefly, you will sign into your correct Google Voice account on a web browser (the account that holds your Google Voice inbound phone number), then open another browser tab in the same browser, and go to the OBiTALK web portal,
https://www.obitalk.com/obinet/pg/obhdevAdd your new device to the portal. Then follow the steps to authorize your device to use your Google account. That's it.