The Definity does not have powerfail lines as a standard. You would need to purchase an Emergency Transfer Panel(808A), which holds up to 8 analog lines. The ETP is what lets the analog lines contiue power when the PBX has lost power. When the PBX has no power, it has no way to let the analog lines power pass through. That is when the EPT kicks in and allows the analog line to reside on the terminated phone. One way we have ours setup is the analog phone has a DID when normal power is running, but when power is lost, the POTS lines from the EPT kick in and we are able to get a hold of the remote location.
I hope this help!