MTN dont cancel contracts and bill clients out

On the 23rd of February, I call mtn as advised the week before by MTN to cancel 24 month contract. I was told the contract was cancelled and later in the day received a mail from retentions to which I rep**** I still want to cancel my contract. They rep**** 2 days later confirming it would be cancelled and ported to prepaid. However today I received a statement saying I owe mtn R869 for monthly R209 contract for usage in March. I called to query this and was told the debit order will still have to be paid despite my contract being cancelled with mtn. I think this is ******** and unfair as I did m part as the customer and cancelled the contract with due notice. I discarded the Sim on the 28 Feb yet I'm being charged for usage? Mtn sort this out before it become a legal matter on ethics and policy.