No replies to numerous emails and phone calls

I purchased an item on credit more than 20 years ago, but returned the item to the supplier as it was faulty. Unfortunately I no longer have proof of this because I only keep my paperwork for 10 years and then throw it away. Now MBD Attorneys are trying to make me pay for it. I have emailed them 5 or 6 times but received no reply. I have phoned them several times and the consultants keep promising to get back to me, but they never do. But MBD keep sending me demands for the money. Today I received a final demand, but obviously talking to them means nothing, all they want is the money. I have made the effort to try and sort it out, but it seems like the only effort that they are interested in is for a person to pay.