Two years ago, someone introduced me to IP-relay.com. It’s not the same as actually making a prank phone call, but it’s still pretty good. For someone like me who can’t do anything without laughing, it’s probably the easiest solution :)
People doing this are also doing these things:
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I used to have this phone number to a telemarketing company, and my friends and I would call them up. They thought they were calling us, so we would pretend to be customers. At one time we pretended to recognize the telemarketer from a gay sex hotline, at another we “fell down the stairs” and hit our heads. It was hilarious. I miss the simple fun of it!

