Oh, mine is so lame compared to yours. But I used my cell phone to pull a prank. Does that count?
My staff are scattered through six buildings through the downtown core, and my bosses are housed in a seventh. Most days I wear my sneakers to hoof it between offices, and I keep my cell phone handy. I call-forward from my regular phone so no-one holds back from calling me. My boss has wondered aloud if I need the cell. I've explained, patiently, that the phone is so that my people can reach ME, not the other way around. Who knows where I'll be that day?
Anyways, this one day he calls me with an urgent request. He has no idea I'm in the same building, working at one of the spare workstations. As we talk, I print off what he needs and head to his office. We keep talking as I walk right up behind him. As he begins to hear the echo, he slowly turns around.
Seeing him jump out of his skin, priceless!
As I laugh hilariously, he sputters and hangs up the phone. I hand him the papers he needed urgently, and do a pirouette, laughing. "You should see your face!"