Amount added to a closed account which affect negatively to my Credit

I have closed my account on the 23 January 2020 and send my proof of payment which I subsequently received a confirmation letter from Edgards that I am not owing my account is closed. To my surprise on the 09 March 2020 I received an sms that I am owing an amount of R300.00 which is for 2 month. The amount is on the purchase of the sms package and airtime. I have never requested any airtime to be send to me. I asked the consultant to check the cell number that the airtime was send to and she refused. I am told that the turn around time for this is 7 working days which is unacceptable at all. This have affected my credit score because according to them I did not pay what was due to them last month and this month. Please remove that amount from my name and clear my records with the Credit Bureau with immediate effect.