Jefferson Capital Systems, LLc said they were attempting to collect a debt for a Credit Card from back in 2006 that I never even applied for St. Cloud, Minnesota

I received a letter in the mail today stating that this company was attempting to collect a debt by offering a payment plan for an Imagine Mastercard ( First Bank of Delaware) card in the amount of 833.56. Here is the issue, I have never in my life had this supposed card and if I had why was it never reported on my credit report until now? I am trying to fix my credit and I find out after I called htis company and told them I have never even heard of the Imagine Mastercard they hung up on me. I called back and asked to speak to a supervisor they hung up yet again. My credit report now shows this debt from 7 years ago that I never had.