I presume you are in the US. Legally, if you did not inquire into, or pay on, this debt in the last few years (7 is the most), they cannot press for collections. But many collection agencies try things that skirt the regulations. Fortunately, they are required to prove that you owe, not the other way around. Demand proof that they own the debt, and that you did not pay it. If they can't prove it, then you can stand on your rights and make them back off. The key is to be professional, not hostile, and tell them that they are in violation of Federal regulations if they cannot prove the existence of the debt, that it wasn't paid off or settled, and that they own it now.