MTN contract cancellation yet still getting billed.

Dear MTN

Your customer care people are not understanding, and keep telling me the same thing over and over.

My contract with MTN ended on 04 April 2019. Payment was done in March 2019 while still under contract. The invoice received in April showed a negative balance of R200 odd. Then MTN tried to debit my account again in May, wich I refuse to pay.

Now your Customer care keep telling me it is because billing happens a month in arrears. If that is the case, why would I be billed a full month of April then? Should it not be pro-rata for the 4 days only.

I refuse to pay the account for May that you are now sending a letter of demand for. Until you explain to me why would April have a negative balance, and then suddenly again in May it is a full month's invoice.

I would appreciate your comment on this.