I suppose the wording "body language" was misleading. It's a mixture of body and vocal language queues. You could both perfect the art in person and over the phone. I don't necessarily agree one is more difficult than the other for they both present different factors of difficulty. In person you are required to perform like an actor. YOUR body language matters just as much as your speech. Whereas over the phone, you don't need a poker face and you can mute the phone and laugh your ass off if you had to.
Peep on James Randi as he exposes psychics and magicians as frauds - mostly because what they are peddling is dangerous - to convince others that magic and supernatural is real which will lead them down the wrong path in life because science and technology is the answer. I hate charlatans.