Acai? review: collections 1

I received a threatening phone call from a collection agency stating that I had a year and a half debt with a company I have never heard of. When I requested that they provide me proof of the debt so that I could review this information and validate the debt. I was told that they would not provide me proof of my debt and that it would be reported to my credit bureau. When told they couldn't do this legally, they told me that I could either resolve the debt with them immediately or it would be reported. I then asked them for a phone number and name and have yet to be able to either confirm the collection company is even real or to reach them via the phone number that showed on caller id. I am furious and don't believe this to be fair! Thsi is against fair credit practices.