The way I see to make it happen is forward your GV calls to a second VoIP provider, CallCentric,
Voip.ms, etc., and use the call forwarding features from that service to ring one or more phones or numbers you have setup.
If yoir Obi in unreachable dur to netcork connectivity it's obviously not going to be useful, and there is no logic in GV Settings that do "if phone 1 is down, then forward to phone 2."