There is also the possibility of using "accord and satisfaction". Once the debt is disputed, if you don't their responses (or their potential counteroffer when you write a letter), you can send them a check for a lower amount with verbiage on the memo along the lines of "full and final settlement" and on the back, under the signature line, along the lines of "by cashing this check you agree to full discharge of obligation on account number XXXXX". If they cash the check and don't refund it within 90 days, that becomes a "renegotiated contract" and if they were to try to come after you for the rest of the debt, you would be able to sue them for breach of contract.

See the below case to get a better understanding: