You chose the perfect example to disprove your own point. Other companies could not run phone lines. There are plenty of hard monopolies, and they’re very different from soft, virtuous cycle monopolies.
Hm, ok, you're right - there was regulatory capture in place at that time, but when the phone companies started running lines in the first place in the 1800's, they had to work around the local regulations at the time as well. Worth noting that the AT&T breakup didn't involve restricting the artificial monopoly, but limiting the power of a single entity to control the market as they had.