UC500.com Cisco CallManager Express, UC500 reference

PFO as shown on the front of the UC500 if for power failure.
When the system is powered off, the black FXO port is connected to the black FXS port.
One strategy, is to connect your main telephone trunk line to the FXO 0/1/0 port. Then connect the FXS 0/0/3 port to an emergency analog phone set.
That way incoming calls to your main line will ring(one at a time, of course) to your emergency phone. As well as making outbound calls.
Also, if, recording after hours incoming calls are of concern during a power failure. A cheap analog answering machine, powered by a small ups, could be connected to that FXS port.

Good luck,
Phil