At a previous employer (large .edu), we moved to VoIP several years ago but keep several PSTN lines -- including all of the emergency ones, such as for the elevator. We could have moved them to VoIP as well but we figured, in a true emergency, it was one less thing to have to worry about.
Most large cities' building codes mandate auto-start/auto-transfer backup generators for elevators in buildings over a certain height. It is sometimes possible to put critical equipment on a generator backed circuit (such as a 48-port 802.3af PoE switch providing power to phones).
