When I was in high school, I used to do a lot of crank calling.
One time, I called someone, with the most ridiculous story I could think of. I told her I was trapped in my boyfriend's car. I made it as convincing as possible, complete with crying and babbling.
She actually believed me! In fact, she started to really freak out, and got out of bed (it was about 2 am) to go in the kitchen and find a flashlight. That, in turn, freaked me out, and I hung up. I felt so bad afterward, I had to call back and tell her it was all a joke. She wasn't too happy about that.