I think you're making things way too complicated.
I have GV setup with Google Chat (which is how it goes to the OBi) and a cell phone. I can check, or uncheck, either one, or have both checked or both unchecked. That tells GV what to ring (OBi and Cell, OBi only, Cell Only, or neither). Regardless of what rings, after 5 rings they get sent to GV's voicemail.
GV emails me when a) I miss a call or b) someone leaves a message, and it sends me the entire transcript including the ability to play the message on my phone.
So, if I am home and my OBi is down when someone calls, I get an email and go "Oh, my OBi must be down".