Telkoms mistake on payment.

Please help me I have a major problem with Telekom. I had a 24 month contract with them for which payment was on a debt order basis. The contract has ended. When a sales reps called offering me a new contract I discovered that there was a balance owing, I contacted Telekom enquiring this but did not get any assist. The account was on debt order basis only regarding payment terms and no other option. Hence I asked the person I spoke to that how is it possible to have missed a payment when it on debt order basis and I never canceled the debt order with my bank. They admitted that they are the ones who made a mistake and miscalculated. When I told them to set up a debt order for the one payment they made a mistake on they refused said it will be too much paperwork on their side, meaning they would have to own up to their mistake and take accountability. Now I'm getting calls from external parties demanding this payment. How is this possible. I need to know what to do to hold Telkom accountable for their own error as they now have added interest on this payment.