It was because of anti-monopoly legislation, https://www.investopedia.com/ask/answers/09/att-breakup-spin... ; i.e. breaking up 'Ma Bell' into the baby bells, and the current oligopoly we have instead in the modern day. An older picture showing how they were broken up and then re-merged: https://www.reddit.com/r/Futurelings/comments/r24ed0/diagram... or https://legalclarity.org/what-happened-to-the-baby-bells-aft...

I had no idea people paid by the minute