Not paying for someone else’s mistake

I have been with Telkom for over 4years. I have not missed a single payment. One off my phones have not been used so the contract ended and I’ve paid it off over the months. I spoke to a lady last month who informed me that I had something like 700,00 left over to pay. She said it was ok for me to pay it over 2 months. I spoke to her on the phone. There should be a recording of it. I paid Last month and and this month I just had a R153, and then I was done. Suddenly I get a message from duke I owe them R1400,00. I phoned Telkom. They explained that because the contract is closed I should have paid it all at once. This is not what the other lady said! He said i just need to pay the last 153, and then phone duke. A lady phoned today. I explained and and said I will pay for a mistake Telkom made. I am so angry she said I am liable, I said I’m not I have paid telkom every month it should not have been at duke! She just kept on arguing in an irritated voice. I can send proof of payment! How is I am to pay for someone else’s mistake.