If you have an old phone (probably corded), connect it on the Line side of the OBi, i.e. directly to the PSTN line. Then, when the cordless system is off-hook, an incoming call to the PSTN number will still ring the old phone. Eventually, someone should hear it and answer, so you can tell them to hang up the other phones.
The old phone may also be useful in an emergency; an off-hook cordless phone, and most problems that might occur with the OBi, won't prevent it from working.
Of course, you won't be able to make or receive VoIP calls from the old phone.
Edit: DaveTap beat me to it.