Struggle to cancel contract

I got a cellphone contract for my 2 children in 2022. It was a buy one get one free offer at MTN. This year June, the contract ended and it went over to pre paid. I have tried 3 times to cancel the contract, to no avail. (I only got it for the phones, we never used the simcards. They had their own already) .. As a last resort, because nothing else worked, I ended the debit order at the bank. Now they want to take legal action against me and give me a bad credit score.. really? Email after email and even 32 minutes on the phone and I just can't seem to get the contract canceled.