MTN handing over accounts withouth your knowledge.

I called MTN to cancel my contract in January 2019 and was told it will be cancelled in February. To my surprise that never happened they debited my account and had to reverse the payment and called to rectify the matter. I was then told that they will have to debit R597.34 to settle the account end of May 2019 and I agree. The debit order was never happened and was not advised the reason and thought all is in order. to my surprise two months later I receive an email from Narton Lambrianos attorneys advising me to pay them because my account has been handed over to them by MTN. I feel process was never followed as I was not given a chance to pay off this account if the money was not debited or advised of the step they taking. I want to pay the amount I'm aware off R597.34 and get done with MTN.