Used to work in a call centre. Made a few needless to say. Not your average prank calls though.
One fella who worked in/owned a furniture shop, was a right tit with his attitude, so called him back a day later. He was in Jersey, so after looking up some businesses, I called him and used the name of the manager of the shop Evans (part of The Arcadia Group.)
Basically had him drive half way across Jersey to 'deliver' a sofa to the store. Called him back the next day to gloat.
Another time, I called a woman, telling her she'd been nominated by 'friends' to appear on big brother. Basically got her whole life story, including details of her sexual relationships and personal life, though I should add that I kept it clean.
Our favourite was to look for 'entertainers' in the Yellow Pages and call them and make them sing their songs down the phone to us, before launching into a scathing attack on their vocal abilities.