In some UK cities, they build hardened telephone exchanges in the late 50s early 60s to try to guarantee communications in the event of a nuclear war. The one in Birmingham is now used for data cables and isn't open to public because of water-table rises.
The Anchor exchange construction project had a cover story: an underground for Birmingham that, alas, had to be cancelled later because of changes in the market!
The main one in London was re-purposed from an abandoned underground station, itself used as a communication centre in the 2nd world war.
There's a building in lower manhattan that was designed for that purpose as well:
https://en.wikipedia.org/wiki/33_Thomas_Street
Technically there are two buildings built for telephone switching, but I don't think the other building (375 Pearl St) is nearly as overbuilt.
The block of flats I live in used to be a UK telephone exchange, and it has some seriously thick concrete walls and massive steel girders. I dare say this was partly to house the old electro-mechanical exchanges, but it does look like it'd survive a bomb blast.