What you need to do if it keeps on happening is what I did with a friend of mine. She kept getting prank calls from a private number, as soon as she answer a guy laughed on the other end and hung up. So the next time they rung I answered instead of her, completely threw them off hearing a guy answering instead, they asked who I was and I just answered with this.
"Nevermind my name, just consider the fact that I'm a 6'1 guy built like a brick s**thouse who thinks you need some maturity and manners beaten into you for pranking my friend all the time."
They hung up and haven't pranked her again, that was about 3 months ago. :D