US telcos as well.

There were several switch failures in the 1980s / 1990s in which systems which had been upgraded in place without a full restart failed. (IIRC, one burnt down, literally.)

Engineers were uncertain as to whether or not a cold-boot restart was even possible.

Account concerning an AT&T system upgrade sourcing Risks Digest (Vol 9, Issue 62, February 26, 1990) by the recently deceased Peter G. Neumann: <https://telephoneworld.org/landline-telephone-history/the-cr...>.