MTN and Nimble

I had a data contract with MTN, and the account was handed over with a balance of R850 because I had stopped working and stopped paying. The attorneys called me, and we set up an arrangement. I settled the account, and they gave me a paid-up letter. Months later I get calls and emails from Nimble claiming that I owe MTN R18K for a cellphone contract. I have made numerous attempts to contact MTN both via phone and email but to no avail. This one time I even contacted ICASA because I had grown tired of the issue. Now I have Nimble harassing me every other day and I asked them to send me the contract that I signed, details of the phone and the delivery details of that phone and who signed for it upon delivery. I have even had to get my lawyers involved in this because this is messing with me big time.