Telemarketers hardly call my house anymore...I used to speak to them in German, or pull the "other" Seinfeld on them (one DannyBoy mentioned)
I did want to do one where I was acting like someone was breaking into my house....but nah.
What's also funny is the door-to-door salespeople. One time, there were these religious people selling some candy for some kind of a church fund-raiser...I played the most satanic music I could at max volume>=)
Also, these girls were selling Girl Scout cookies (oldest was like 9...) so I scared them off by wearing this Halloween mask I still have. Scaring youngins' is funny

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