Mislead into thinking my account was paid with their stupid 3 months special

This company called me some time in June to give me a 3 months special to settle my account. They told me I need to pay R650 for 3months to settle my account which I did, however every single day I am being harassed by their call centre agents telling me that I still owe them and I did not pay. Every week I find myself having to explain to every call centre agent. They don’t keep track of the previous conversation. They are ***********!!! SEND ME MY PAID UP LETTER !!!! I AM DONE PAYING YOU !!!!