An 'aside' on Exchange power arrangements
I have no idea what arrangements are/were made for 'power security' on the U.S. telecommunications network, but here in the U.K. all the exchanges built/upgraded during the 'Cold War' era were provided with a Central Battery which could run the equipment for 24 hours in rural areas, and 1 hour in urban areas. The generators had fuel enough for 7 days running at full load, before a delivery was required.
Unfortunately, the modern 'more efficient' digital equipment has a higher power demand, most of which cannot be supplied from 51.5V battery. Our local 'main' exchange (which also houses the Group Switching Centre, and most of the 'trunk' lines) was in the process of being 'upgraded' to digital when I left the company. The old 560kVA genset was being replaced with two 800kVA sets, to accommodate the extra power demand of the new A/C system required to keep the new equipment cool enough to work. The two sets were to work in parallel, with only partial redundancy. One set COULD just about run the load, but only with all non-essential equipment switched off. Under the 'old' system, there was power enough to run EVERYTHING in the building, and some to spare....
Newer digital exchanges built/upgraded during the late '80s and '90s had only minimal battery backup from 'sealed' gel-type batteries, designed to support the load for just long enough for the generator to start. :-(
All best, and thanks for a fascinating thread
Dave T