@Carl Don't give up! This forum and the folks here are great at helping almost anyone get something up and running but sometimes (if you're like me) you need to visually see the steps to something in order to get it going. Might I suggest if you're still having trouble and it's just not working (an you indeed want OBI and GV to work) then perhaps you go to
youtube.com and in youtube's search bar type how to setup obi and google voice. This will produce quit a few video how to's that might be of great help to visually understand what steps you might be missing in your setup attempts. Take a deep breath, put on your big boy pants and give it another try!
It seems you were stuck on "forwarding number" don't let that confuse you.
Initially you need a forwarding number but you can uncheck that at a later time
and just place a check in the google number box. I have the very same setup that you want, a google voice number, and OBI, a cordless phone...I do not forward any calls to a number other than my GV #. When someone calls my GV # my cordless phone rings and I answer. Same for outbound...I dial a (US) xxx-xxx-xxxx on the cordless phone plugged into the Obi and it calls with NO CHARGE. Carl, give it another try!!!