Hell. Even Twilio (a common carrier) banned Parler. AFAICT Parler wasn't using Twilio to forward harassing messages. It used it for 2FA. And I'm sure they'll get away with it. It's just nuts.
https://twitter.com/twilio/status/1348719143172927491?s=20
Twilio said the Parler platform was in violation of the acceptable use policy (not just the use of Twilio services), and was told by Parler that they had already disabled Twilio.