Still being invoiced for a cancelled line

I cancelled my Telkom ADSL line on 9 July and received an SMS acknowledging my cancellation request. I have not used the line since 9 July as I moved over to LTE with another company. To cover any cancellation period I paid the following two invoices I received. On receiving the third invoice I emailed Telkom. I have now received a forth invoice as well as being charged interest for not paying the third invoice... on a cancelled line... which they acknowledged would be cancelled on 9 July. Do I now have to get a lawyer involved before my credit rating is tarnished due to Telkom not processing my request?