If you’re going to have phone sex, then you have to enter into a fantasy together. It generally helps if this mutual fantasy is based on something real—something that actually could happen if you were together. So it is a natural question to ask a person what they are wearing and also where they are and what they are seeing right then.
It’s a set—realistic on one hand, imaginary on the other. You hang the plot of your mutual fantasy on that set. It’s not a silly or embarrassing question. It’s an information finding question. Based on that question, you can take any steps you want in the scenario you create.
Phone sex, and this may sound odd, is not about phone sex. If you think that you are having sex on the phone, you’re going to feel ridiculous. However, if you are creating a story you both believe in and you both help create for each other, then it’s easy to imagine yourself actually in the presence of your lover, instead of alone.
You close your eyes and you have your lover’s voice purring in your ear, and hearing “what are you wearing” is not silly at all.