When the Bell System was trying out prototype ESS gear, they had a real central office served by their new switch. They had to make some changes and planned to do so at 3am. However, they hesitated about taking the switch down at that time, since if a call did come through, it likely would've been an emergency. Sure enough, a call did come up and it was indeed an emergency. The engineers realized 24 hr service had to be truly 24 hour service.
I don't know about today, but ESS was originally built with two CPUs. One was handling calls and one was reserved in case of failure or to do maintenance. Early ESS turned out to be extremely reliable -- the longest outage was on account of a failed air conditioner. (IIRC this was the Morris IL test).
One of things the big Bell System was able to do was 'beta test' new hardware and software under very controlled conditions before rolling it to the whole country. A small community would be selected, residents notified and trained accordingly, and the system tested. Not everything passed the test -- certain call features weren't popular and their initial tone ringers were disliked. The experience the engineers gained from seeing and maintaining their switch in real service serving real people was invaluable.