I hesitated posting this before, because the first comment was bound to be, "Yeah, but Obihai should have just left in the relay" and so on. I agree, but here is a fairly simple solution using a relay and a wall wart for those who don't mind hacking a solution.
This relay has screw terminals and an LED. It also works on AC or DC of various voltages so it should be easy to get a wall power supply to help. Then a couple of RJ11 jacks and a case. Not cheap but pretty easy to hook up.
http://www.azatrax.com/track-power-relay.htmlWithout testing, looks like you would extend the phone line to also connect to 1NC and 2NC. The phone goes to 1C and 2C and the output (phone port) of the OBI goes to 1NO and 2NO. With no power, the phone connects to the NC terminals and the phone line. When power is applied, the phone connects to the output of the OBI.
The relay would be powered from the house power for automatic use, or with a battery or two and a manual switch for manual use. Of course, if you want to manually trigger the device, you could replace the relay with just a DPDT switch at a cheaper price.