Still being billed after contract cancelled!

My contract expired in November 2025. We returned the device to the nearest MTN store as requested. We filled in the forms and were told everything is sorted, the contract is canceled. We were then billed again, so we contacted the customer care and they told us we must go to the store and get an IRMS number, which we did get and sent it through to the customer care. However they are still billing us every month although we don't have the device any longer or access to us the service any longer. They are *******ly charging us for absolutely nothing because now we don't have the modem and simcard as it was returned to MTN. It's anightmare getting hold of a human being on their customer call.