Back in the late 1960s, and early 1970s, AT&T built a series of nuke resistant bunkers. The typical specs were concrete walls 36" thick, self contained turbine generators, water and air purification, plus rations to last up to 6 months. Some of those facilities are still around and operating (including one not far from me). I would expect that the switching gear of todays vintage takes up much less space and power than it did in 1971. Back then the interconnections were via long-distance coxial cable and microwave. Today there are fiber huts adjacent to the bunker.

I wonder what AT&T uses all that extra floor space for now.