I work at a Subway inside a college student center. There are courtesy phones on the other side of the foodcourt. It's fun to call them from our phone, wait til some random student picks it up, then just say some random ass things, then watch the reactions.
My favorites so far were:
*Adopting cholo accent "Yo mang, drop off is Taft and La Porte(two streets that intersect a mile or so out of town) at 11. You be there ese. You ****ed up last time, this time we don't forgive"
The guy's face went pale white. It was also that time I realized maybe I should tone it down a bit :x
Another time:
Code: Papa, Echo, November, India, Sierra. What is your report?
The girl who picked the phone up that time just walked away shaking her head.
We have a problem with vagrants taking up shack in the student center, hiding at night from the janitors and just sitting and watching TV all day while begging us for food. Normally, most of us don't care so long as they aren't hurting anyone. But there is one obnoxious guy who's been kicked out multiple times, but always manages to return. One afternoon, we realized that he had probably given out the number to the courtesy phones as his own, because if someone called, he would go over and answer it, then talk for hours on end.
One day I called it, watched him run the 30 or so meters the phone, then hung up as he picked it up. He answered, got dead air, then returned to watching Gunsmoke. I repeated this five times before he gave up. This is fun to do to this day, but usually he'll only get up two or three times now.
I'll also call the other two phones if the vagrant is talking on one, just to annoy him.