If (and I know is a big if) the phone itself doesn't need power, calls could still be made during blackouts.
As long as you have the Obi110 with a relay. Either unplug the power and plug it back in, or simply reboot your unit and listen for a click.
This is an unconscionable bug, not a feature request. 911 stops working when you need it most? unreal. I hope Obi fixes this before the word gets out.
For this reason alone I stopped short of redirecting all my phones through the OBi. One thing to remember, you cannot use most cordless phones during a power failure. Always have one old corded phone directly connected to your PSTN line (if you still have that service).
My 110 is the original 2.8 with the relay. I also connected a single phone ahead of the Obi directly to the PSTN for two reasons. I wanted to continue using my answering machine for the PSTN line and also for backup in case of power failure. The hardware fail over is a nice-to-have feature for me but as many have noted, your cordless phone is dead during a power outage anyway unless you have battery backup for it.