Dispute with MTN.

I had a 24 month contract with MTN. After the contract ended I send an email to *** to notify them about the cancelation of my contract. I then received an automated response. I had no other response after that. I saw that the debit orders went off. I then returned to the bank to reverse the debit orders. After a few months I received emails and phone calls from nudebt management a debt collectors company to pay the amount which is behind on the contract. I then made it clear that I'm not paying a cent further.