Att sent a fraudulent claim to ERC collections

I got a collections letter from ERC 3.2.17 stating att sent a $1900 collection to them for an account I did not open in 2/16. I had att but settled and cancelled my account in early 2015. I called ERC to inquire and they said they'd send me a fraud packet (which I have researched and no one says this helps). I called the 8003310500 att number and they stated they didn't have any information to help me and suggested I go to a att store for help. I need answers. I did not authorize two accounts to be opened in 2016. Att never contacted me about a bill and the first I've heard about this fraudulent activity was when i got the collection letter. Somehow ERC has my correct address but att doesn't? I don't understand what's with the non existing communication. All I'm reading about ERC is how they buy a lot of fraudulent claims and hit people's credit score hard. I need to clear up this situation before it gets on my credit report. I have an 800 score and have never been late paying anything. How can you help me ASAP att? The fraudulent account was a go phone. Bdixon@theimpactanswer.com