unauthorized debit orders

I have a contract with Cell C (cellphone) that expired in Jan 2022, however, i am still being billed and charged? how is this possible? the contract duration is completed but I am still being billed and also NOT receiving any data or minutes? so I am paying over and above and also unauthorized debit order for a contract that's completed? i have called the call center numerous times and i have not been directed accordingly, I am not happy with this, and no communication indicating as to why i am still liable to pay?