being stalked for the payment

I took a contract with Mtn in 2013 no longer sure about a Month. Then a nanny I had got hold of the Contract simcard which I was not using and used it. I got a statement saying I was owing R2500 sum change. I called Mtn to report the incident and promised that I will make payment towards this accounts meanwhile both my contact sim and a simcard linked to contracted were already closed. In November 2014 I called Mtn to settle the balance, they gave me the settling balance and I paid the same day and email my proof of payment. To this day I am being harasses by calls and messages form Norman and Associate saying I owe Mtn R7878.81. I have call in numerous of time speaking to different call centre agencies but to this day I have not received proof that I have paid. I have wasted so much money calling Mtn and Norman &amp