Unending Invoices

Hi My name is Thandeka Mkhize,I had a Contract with Telkom for two years and I cancelled it End of August this year,it was cancelled on the 2nd Sept according to the phone I made to them to confirm and last month in October they sent me an invoice saying I'm owing them the subscription money mind you I haven't been using their services since The 1st of September and the simcard is not working showing the service is cancelled but I'm still recieving the invoices,I called last month asking about this and the call center agent told me it's because I owed R20.50 which I paid,I don't know why they still sending me these invoices.