I think that the relevant laws are those related to the phone/cable companies being public utilities (and thus explicit, by-design, state-permitted monopolies or duopolies). They aren’t allowed to wiretap them (because communications privacy was a bigger deal to legislators pre-internet) and have to provide service to all comers (ostensibly in exchange for being a monopoly-by-design).

From my limited understanding, this regulation forcing them to offer service (as a utility) to 100% of the market is coordinated on a state-by-state basis by the public service/public utilities commission.

https://en.wikipedia.org/wiki/Public_utilities_commission

(Fun fact, I learned this at a young age because my dad ran a paging/voicemail service out of the basement of our single family, suburban residential home when I was about 10. We were the only house on the block with dozens of trunk lines coming into our little bungalow; but by law they had to do it if you ordered it. Try that today with internet access from a cable company, ha! It’s all but impossible due to TOS to run an internet business at a residential address now. Hosting for-profit services with the internet you pay for or reselling the service in any way means you get instantly unplugged.)

Sorry I don’t have a direct link to the all-comers bit of PUC/PSC regulation, but this should give you a starting point for research.

The not-allowed-to-tap-phones bit is a federal law:

https://www.law.cornell.edu/uscode/text/18/2511

It’s sort of insane how provider-wiretapped has been the all-encompassing default for almost all of the largest DM/1-to-1 communications systems in the world: SMS, WeChat, Facebook, VK, Instagram, Gmail. WhatsApp and iMessage are outliers in this regard. Almost all popular new entrants like Slack and Discord are provider-tapped, too.

This is a relatively recent development in our society’s relationship with electronic communications. Reading content by the provider used to be illegal as fuck.