I found this old ATT promotional video from 1967. It mainly concerns the PSTN wired network reliability.
It’s a mildly hoaky propaganda piece, but we often forget how our modern communications infrastructure is very young. If ATT seems proud of their system, they had a right to be. Prior to electronic communications, you wrote a letter and hoped that it arrived.
There is also this video: What is the Bell System?
If you have the time, there are many interesting archive videos on the ATT Tech Channel. Be careful, one can spend a lot of time watching these videos!