Pick one friend that you are going to abuse. Every time you call them at home, pretend that you cannot hear them when they speak.
"Hello? Hello?" *click*
Everytime they call you, do the exact same thing.
You can always add in a few lines such as, "I don't know who you are, but this isn't a funny joke."
If you see them on the street, act suprised and relieved.
"Oh that's YOU calling me. Here I thought it was a crank caller!"
Usually they will blabber out some apology and all the time you can smile to yourself knowing you are really pulling one over on them.