You're using Google Voice backwards. It's designed to forward calls from your Google Voice number to your cell phone number(s). Add and verify your cell phone number(s) as forwarding/linked phone numbers.
You then set up conditional call forwarding on the cell phone, to forward unanswered or busy calls to your Google Voice number.
Then, on the Google Voice Legacy settings page, Phones tab, click "show advanced settings", and then, on the next page, change "Forwarding options" to "Ring my other phones before going to voicemail ". Save your settings.
If you don't want to use your cell phone while you are home, then just turn it off. Calls made to the cell phone number will forward to your Google Voice number, which will forward to your OBi device. If you don't want to answer the call, it will go to Google Voice's voicemail.