Someone correct me if wrong but I think phone co's can get away with it as they're deemed an information carrier instead of a host. Besides, calls and texts are rather benign, it's visual media that tends to attract the most calls for censorship, not to also mention that real-time censorship of text and voice would be infeasible but much more viable for an information/media host.
And phone companies don't play an editorial role in determining what calls you get to hear, and who hears yours. Newspapers are responsible, even when they're publishing someone else's submission to them.
That is precisely the issue. Phone companies don't maintain control of their network to filter what calls and texts are allowed, don't maintain political rules on their network, etc. They are neutral carriers.
Meanwhile google etc are pretending they are neutral carriers while actively filtering and choosing what is shown, now politically, showing they not only have the capability to control their platform but put it in practice.
If they cannot allow others free speech they should not get special protection that prevents their liability.
If a phone company started filtering political discussion I would absolutely think them responsible now for everything on their network. Your example is perfect to show how different both groups are in capability and practice.