Phone companies already ban spam customers that make thousands of phone calls

That's according to volume of calls, not content, so I would assume it's covered by different legal theory.

Banning based on complaints by recipients of the calls is also different from banning based on some sort of automated monitoring without any complaints.

Kiwifarms generated complaints.

censorship is all about power so the focus is on controlling any mechanism of broadcast communication that could result in political organization. traditional voice communication over phones is very inefficient compared to internet based text communication so it would probably be a low priority by comparison.