One night after a bit of partying and a dip in the hot tub (the dorm I was in was converted from a hotel) my friends and I were in the laundry room drying the clothes we were swimming in, and noticed what appeared to be a closet in the back. Opening this door revealed an airconditioned room containing routers, ethernet hubs, and the telephone wires for about half the building. I amused myself for a few minutes unplugging people accessing the internet, but then, being drunk, my friends and I had more fun seeing who could fit themselves inside the industrial clothes dryers. Even at 6' I'm the only one who could get in and close the door all the way.

But anyways, later, when I was sober I realized there was great potential in that little room. I noticed that all the phone jacks in the building were numbered, and followed a pattern. It only took a few random samples from other rooms to allow me to map out the entire building.
The original plan was to switch all the phones around randomly, but we realized this would take a great deal of time and would be caught almost immediately. We settled on a more subtle approach, we would target a suite, and rotate the phones between the individual rooms, so that each room was now one number off. This way it would take much longer for the victims to catch on.
Our targets were the girls next door, whom we knew, so we would most definetly hear of the results of our prank. So at 3am one Thursday morning, in a very Mission Impossible like manner, one of my roommates and I sneeked down to the laundry room, equipped with needle nose plyers and latex gloves. Upon closer inspection of the phone connections however, it turned out that our neighbor's room was not hooked up in this particular room, but in another (locked) room on the other end of the building. We moved on to our secondary targets, the girls immediately below us, who were much more deserving anyway, always complaning about how loud we were and banging on the ceiling. I only switched this one room because sometimes college students will keep odd hours and it's not unusual for one to do laundry at 3 in the morning, so I was in a bit of a hurry.

I don't know how well this prank succeeded, because we never really talked to the girls downstairs, but I can only assume they eventually complaned about it because I went back a month later and found the little room locked.