As far as phone companies, my current understanding is that in the U.S., the government is involved in building and maintaining telecommunications infrastructure. Therefore it falls under the First Amendment.
If you run your own wires between a lot of houses to let people talk to each other, and then listen into every conversation and cut off ones you don't like, that doesn't seem illegal.
Things like Discord or Slack, for instance, should legally be able to moderate anything you send through them. Though it would be financial suicide to actually do so.
[Disclaimer here that I only have a vague understanding of telecommunications infrastructure]