Prank calling telemarketers taking orders for late night commercials is always fun (especially the Girls Gone Wild numbers). I used to work as one and got several prank calls...and I would mess with them back.
One lady called me about a steamer and asked if it's safe to use on her dog...I could tell she was joking because I heard a giggle in the background, so I told her..."listen...why don't you order it and test it for yourself on his ass...if he doesn't complain, use it on the rest of him" and I hung up.