A solution to the emergency phone might be to place a 2-way splitter on the POTS side of the Obi so that the pots line splits to a phone AND the Obi. The phone would have the ringer turned off. This phone could always be used to dial out (if desired) or in an emergency. While not as convenient and any other phone in the house being able t odial without power, it would suffice.
Simple external relays could be used to switch the POTS line to the phones in the house to bypass the Obi if necessary and not have to open the Obi.
If you were just trying to switch between the Pots line going to the Obi or an external telephone, a single DPDT relay would suffice. However without designing a solutions, my first thought is two DPDT relays are needed because you need to switch both pots wires (and I don't know if there is a common path inside the Obi with no power) and you also have to open the phone side of the Obi and not apply ringing voltage to the phone port.
(Note: As I thought this out, I looked up a 4pdt relay and designed this. It is untested but looks like it would work)
120 volt 4PDT relays are fairly cheap although you might be able to find cheaper ones at dc voltage power them with a wall-wart (power supply) if you have one to spare. We are talking perhaps $10 for the relay, maybe a bit more, some wire, some RJ11 jacks, solder and a small plastic box.
In this diagram, when there is no power to the relay, the pots line comes in, goes to the Common terminals and because the relay is NOT energized when the power is off, the NC connections are used, bypassing the OBI. When power is applied, the POTS phone line goes into the Obi and back out to the house wiring.
Sorry for the crude diagram, but it's been a long day.