One place I have worked at they had an older phone system. You could change the person's name of the extension from the handset if you knew what you were doing. I did know how to do this and scared a (not very blameless) colleague sh!tless by changing mine to display the name of the Chief Executive. I called his number and the poor bloke visibly jumped when he saw who was calling. After realising he was being set up he wanted to know how to do it. When I got in the next day my phone was telling people that I was Mr Tickle. Apparently I was going to be something much worse until he realised my retaliation would have been much worse. He didn't want to escalate the situation further as he'd started it by signing me up to the Britney Spears fan club email news. This was before verifying your email became a popular thing.