This was my original thought, but I don't think so now.
The only reason your cell wouldn't ring is if CC answers the call and forwards it to the Obi.
As QBZappy suggested, you can fork calls from the Obi to your cell phone.
You could also set the Obi to forward on busy to your cell phone. This will not interfere with your google forwarding because when CC answers the call, google will be dropped from the call process. Of course you will have to disable call waiting for the phone port in order to forward on busy.
Perhaps there is a setting in CC. Do you have it set for forwarding on busy or anything.