Incorrect Billing

I would say shocking experience but I am not this is typical MTN service. I purchased a month-to-month LTE contract on 26/12/2023 for R499, the device was delivered on the 30th and activated on 4/01/2024. I paid R563.39, pro rata plus Jan subscription. End of Jan R917 was debited instead of R499, I paid it no mind because I thought that it would cover 2 months, February and March. Now I am getting a statement that I need to pay R499 by the 4th of March. How is that possible what was the R917 for because I signed up for R499.