Doing actual actual crimes or committing a fraud using the phone, computer, fax, telex, or whatever other electronic means generally falls under wire fraud.
The phone company doesn't de-platform "literal nazis" simply for being "literal nazis."
The distinction is between perfectly legal speech, and specific acts fraud or things that are direct, actionable, and credible threats to physical safety (e.g., bomb threats). This isn't a complicated blah blah, where we need a technocratic meta-god to put his thumb on the scales. The existing legal pre-computer legal frameworks cover these cases.
And if the literal nazis we’re actually using a private person to person (can be through a service) communication medium like a phone the number of people who want them deplatformed would drop to basically zero.
Your argument has to account for the fact that Twitter is more like public radio than a phone and that WhatsApp is more like a phone than radio.
To be fair I do see articles written pointing to the fact that hate groups are using signal or discord, the implication being that such companies are "platforming" those groups.
The phone company doesn't de-platform "literal nazis" simply for being "literal nazis."