NCO Financial Services sent bill for erroneous debt that I disputed over 3 years ago Trenton New Jersey

I just received a collections bill from NCO Financial Services for over $600, for an account with Niagara Mohawk. I have no knowledge of this debt, and in fact I disputed it over 3 years ago for the same reason. Experian, Equifax and TransUnion investigated and the item was removed from all 3 credit reports. I see on this website that there are many others like me who are dealing with the same issue with this company.

My question is, since there are multiple addresses for this company on the notice they sent me, to which one do I send my dispute letter (certified mail, return receipt requested)? I don't know if you can send certified mail, return receipt requested to a PO Box. There are 2 different PO Boxes in Trenton, NJ listed on the bill they sent me. There's also an actual street address for Horsham, PA. Should I send my letter to their office in PA? Any advice will be greatly appreciated. It stinks to know that these shady companies can do this over and over again after the debt has been deemed invalid.

womynist
Nashua, New Hampshire