Here's a script for fun and amusement, especially designed for use over the telephone.
All you need is a sub whose posthypnotic abilities include "freezing", verbal control of pain levels, and the ability to orgasm on command.
It also helps if she craves pain; the scene will last longer.
"Pinch your nipples. Hold your fingers there. Don't squeeze any harder, just leave them like that.
"Now as you count slowly out loud, from 1 to whatever, the pain in your nipples will rise accordingly.
"Say "1" ... now say "2" ... now say "3".
"See how it works?
"OK, a couple of things I forgot to mention.
"Your fingers are stuck there. You can't remove them.
"And you must keep counting, until you orgasm.
"Go ahead. I want to hear you count."