Non or bad customer service

My data contract ended 30 Nov 2019 . On Monday 6 Dec 2019, I called in to get the contract cancelled end of Dec 2019, an SMS was sent to me confirming case ***9 has been logged for service ***, however in Jan 2020 I received a bill for Jan even though I did not utilise the data I was charged for the month of Jan 2020. This morning I was debited again. I will stop the debit order until my billing query has been resolved.