Cancelled contract but still billed.

My contract ended in November 2021 and went over to a month-to-month. I cancelled the contract in December, to which the sales consultant informed me that I must give one month's notice. This was done and I EXPLICITLY ASKED when is the last day that cell c will debit my account. I was given a date and also told I will be billed pro-rata for that month (1 day). So I cancelled in December, went over to another network by Feb, but the following month (March) my account was again debited by Cell C. I cancelled this debit order on my side. Cell C is now demanding another month's payment, and each month placing a non-payment on my credit score. This even after I phoned numerous times, and the consultants / accounts confirmed that I should not have been debited. They even confirmed the telephone recordings and the date confirmed to me on previous phone calls to them. The accounts department however just refuses any correspondence and says I still owe them. I also requested that someone from Cell C correspond with me, but this never happens. So what is next, do I just get negative scores on my credit record each month? Guess I need to get the lawyers involved.