My mate had a setup as a teenager which allowed him record crank calls straight out of the phone onto tape. We were listening to some of them last night, as he's back to see his parents for a bit.
Some of the calls are hilarious, he did a whole set using a guitar digital delay pedal, calling people up then having them talk to the echo of their own voice. One victim even says "Your impression is crap, doesn't even sound like me!" thinking another person was repeating what he said, haha. Sometimes he would get the phone numbers for a local phone box by the shops, then ring them and wind up the kids hanging around there: "This is the police, we can see you smoking weed!" or "This is Jesus, I love you all, meet me in the church over the road", etc...