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]
Slack and Discord have self moderating. You can create your own discord or slack server and kick/ban whoever you want.
Reddit relies on self moderating your own sub-reddits. If it didn't then sub reddits like /r/sino prob wouldn't exist. But you have to go and search/find sub reddits or navigate from click-bait titles on the homepage.
This is unlike Twitter where it's all public, people you follow who like things are thrown in front of you, and all misinformation, propaganda, violence, insults, whatever is shoved down your throat, and the public cannot determine what should and should not be allowed on there, its up to a private company who is looking to 1) Profit, and 2) Protect its own agenda.
It would be nice if we had a balance of platforms, but unfortunately we have 1 for each medium. Facebook, Twitter, Instagram, Tiktok, YouTube. Each of these have not 1 real competitor.
Social media platforms are content-neutral. They want to be treated like the "public square" because a lot of benefits come along with that. But then at the same time, they censor content that they disagree with. That's where it crosses the line. If Twitter came out tomorrow and declared that they are officially a left-wing social network and updated their rules to reflect that, then I'd have no problem with them kicking every Republican off the platform and banning every person who complained about taxes being too high. But Twitter won't do that.
I don't want to get further off on the tangent about telecommunications infrastructure, so I'll just leave that alone.
That being said, after having a political/ideological group I really liked removed off a major platform (that it followed the ToS of) without analogous opposite-side groups being removed, I'm at least very sympathetic to your position. I thing we both agree that harmless differences of opinion shouldn't be removed, but I'll agree to disagree on whether the law should be involved.
Well that's always the million-dollar question, isn't it? Take Twitter for instance. Whether there's an easy alternative to Twitter depends entirely on your reference class.
The social aspect makes this so much harder as people are unique. It's completely reasonable for someone to need to be on the same platform as some other particular person. Say, Elon Musk. Or pre-ban Trump if you want the least convenient example from a civics perspective. How do you justify kicking someone off a platform for having the wrong views if that's where their elected representatives spend most of their time?
But this perspective completely defangs your position in all but principle. Any privately-owned platform with important[1] people using it is basically any platform that has any reasonable amount of success. Even if you restrict to public officials, well, there's a lot of public officials.
[1] Note important is in the eye of the user, and people will generally self-select into platforms that have users that are important to themselves.
Just because some "famous" person uses a platform shouldn't mean the platform has to fall over itself to make sure their every word is hear and everyone can contact them.
That's not quite what I said, but yes, kind of. No phone company is allowed to moderate the speech that goes over their network, for example.