Collection agency refuses to show proof of debt.?

My father in law got a phone call in 2006 from a collection agency saying he owed them money. Since he does not speak english my husband spoke to the guy. He told us that he was collecting a debt from 18 years ago from a hospital in Florida. My father in law has never been to Florida. And he was not in the country ... show more My father in law got a phone call in 2006 from a collection agency saying he owed them money. Since he does not speak english my husband spoke to the guy. He told us that he was collecting a debt from 18 years ago from a hospital in Florida. My father in law has never been to Florida. And he was not in the country in 1998. The guy just laughed a smarmy laugh. So a week later he sent a legal letter informing of the debt. When we wrote back disputing the debt and demanding proof we did not hear back. We thought the matter was settled. Apparently not. He started calling again 2 months ago. He is insisting we owe this supposed debt. We told him he better send proof and he never did. But we get electronic phone calls with messages to call him but he never picks up the phone.He calls 2-3 times a day. He won't give us his address so we can send a proof request. It is very annoying to have to go get the phone just to find out it's just this jacka$$. What else can I do? P.S. It is only $400.00 so this is not about the money. It is about money we DO NOT owe.
Update: Thanks everybody. I will do a reverse google and get the address and send them a cease and desist letter.I have the first letter somewhere, and yes I sent it certified. I am going to contact the authorities and report all the laws they broke. Everybody get's best answer!!!
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