Aggressive Collector Theatens to Sue

I have in the last 72 hours been dealing with an aggressive debt collector. I first received calls from a document service stating there are legal documents that need to be signed for. She did not give her company name and the number was an IP number. She also gave a phone number that I can call to get more information. I call the number and it is to a mediation group that is filing a lawsuit on a debt from 2011. It was stated that I owed $3600 and they sent a letter to my parents 60 days ago about it. I haven't lived at home for 2 1/2 years and my parents did not tell me about such letter. We asked for another letter and he refused to send it. He said he could settle the debt for $1936. The debt was charged off in 2012 at $2022. He gave us 48 hours to decide otherwise he would proceed with the suit. We asked twice for written verification but he refused to give it saying he already sent it. His address is a PO box at a UPS in CA. Trying to see if this is legitimate or a scare tactic.