Thought I'd share this video of two people exploring a cold war-era bunker with a surprisingly still intact telephone exchange, granted it's been vandalised, but a lot of it is still there and probably serviceable once cleaned up:
https://www.youtube.com/watch?v=Gwed0aR4ECk
Mostly Strowger step by step
You may see some frome a Norwegian cold war exchange from the Cuba Crises 25 sec int this film: https://digitaltmuseum.no/021115517241/asen-nodsentral-innlemmet-i-norsk-telemuseum
The complete manual backup, the exchange is protected as a museum.
It's remarkable how fast such places have deteriorated. It's difficult to locate any of the facilities I worked in during the late '70s to 1980. I know where they are, but they are either put to different purposes or obliterated and a part of agricultural fields. I hate to se the memories gone, but it's good they aren't needed now.
From 20:02, another cold war-era exchange gets a look, and this one appears to be somewhat functional:
https://www.youtube.com/watch?v=FdLoUF3CBU4
Though not in an MOD bunker, it's another cold war era exploration of an abandoned site (given the US power sockets it was probably joint UK & USA operations), telephone exchange mostly still intact at about 14 minutes in:
https://www.youtube.com/watch?v=rWLbn2OhMoc