can my e-mail service, my phone provider, UPS or Fed Ex censor the content of my correspondence?
And yes, phone companies reject phone sex lines or anything else they might find objectionable. Recall that banks have declared they are closing the accounts of gun store owners - that is a form of censorship and they have the right to do that. And some banks have said they won't be processing credit card transactions of gun manufacturers/gun store owners.
It is a settled issue for centuries. Only the government is subject to the 1st amendment, not private companies. Private companies censor all the time - from prohibiting what their employees can say/do to banning people they don't like on their social media platforms, as FreeRepublic has done and will do in the future. FreeRepublic censors.