Should I call or text for this conversation?

I’ve been seeing a guy for over a month. We are not official. He wants sex. I have a fear of vulnerability and fear if we have sex he will ghost after. A boundary of mine is I will only have sex when in a relationship. A few days ago we were intimate and he said he wanted to fuck, I said not today because I didn’t want to have that conversation in that moment. I understand having this conversation in person is probably best but I don’t want him to just end the hangout and kick me out. My issue with talking over the phone is he may feel put on the spot and text would give him time to think out an answer. Which should I do?