I think you have a misunderstanding of how Google Voice works. It's not a free telephone company; it's a call-forwarding service. You have an inbound Google Voice telephone number. That number can forward to up to six different 10-digit US landline or mobile phone numbers, and it can also forward to the old Google Chat (XMPP). OBi devices are acting as Chat clients. So, your OBi device is just another forwarding destination, in the same way as your T-Mobile number (they are peers).
GV forwards calls to all linked/forwarding destinations simultaneously. There is no "rollover" or hunt group function. GV is forwarding the calls directly to your cell phone, (not to the OBi, such that the OBi would forward the call to the cell phone number).
By default, GV will send calls that are unanswered in 25 seconds to GV voicemail. If you have correctly set up CCF on your T-Mobile number, and you don't answer the call there, it will go to GV VM, too.
If you don't want your inbound GV calls to ring on your OBi and/or on your cell phone at night, you can set up a ring schedule. Using a laptop or desktop computer, go to the old, Legacy version of the GV website, go to Settings->Phones, click the Edit button, click the "Show advanced settings" link, and set your schedule.