Phone sex conversation may take many forms, including (but not limited to): guided, narrated, and enacted suggestions; sexual anecdotes and confessions; candid expression of sexual feelings or love; discussion of very personal and sensitive sexual topics; or just two people listening to each other masturbate.
Let's say you converse with a total stranger over the phone and you moan and you talk dirty things and the person follow suit, you get high and release your loads. Afterwhich, the other so happens to be a ugly being or someone overly obesed. Isn't that gross? Or someone you just conversed with happened to be an old lady instead of a young girl.
Its better face-to-face, at least, there's emotions, and physically present. You can feel the other party.