Anybody remember intensive and extended field tests?
I do.
I was in the Telephone Central Office replacement business in the 80's...
Go to some town that had a perfectly adequate working analog phone system for their thousands of customers.
Install and test a Japanese (NEC) digital replacement. Max size 100,000 customers, or 60,000 inter-office trunks, or any mix.
When the time came, it was called cutover - cut all the subscriber cables going to the old gear, plug in the paralleled cables going to the new machine, push the button and see if it really worked. There was no going back. It usually didn't get too ugly. Had to work, there were no alternatives.
That morphed into working with data backbones in the 90's, which was not so labor intensive.