What should I do about this?

Wow, that's quite an episode. Firstly, I don't think he's upset with you. I think he's embarrassed of what happened because he didn't hear you on the other end of the line. He may have thought that you were thinking how weird he is or thought that you hung up on him. I think it's best to clear things up right away. It may take a while for him to get over his embarrassment & call you back, but it'll happen. Just be sure to continue texting/calling him every other day so he knows you still want to talk. You've got to be honest with how you felt about the phone sex incident (turned on, turned off, grossed out, interested, you want more, you don't want more- whatever) the next time you talk to him. It'll be a comfort to him to know what you were thinking because his imagination is probably coming up with things ten times worse than what you're actually thinking. Let him know that you're still interested in him regardless.