Cancellation nightmare

Iv been trying to get hold of Telkom to cancel my account after my contract had ended. I do not stay at the residence anymore. I have also changed bank account so my debit order has bounced twice and Iv been trying to cancel my account and pay the last payment. But nobody is getting back to me and I cannot cancel my account online because there is a payment to be made. The amount to be paid is R1200 yet my last statement says I owe R2600? I don’t want to roll into another month owing more money when all Iv been trying to do is cancel my account because I no longer live at that residence and am no longer using the service. This has been going on since January.