Failing to calcualte accurate installment.

I have been calling Edgars call Centre every month for them to try adjust the monthly installment as I am suppose to pay around R500 but they invoice me R1450 per average per month, when I spoke to them first time they told me its because I am purchasing airtime using the Edgars account so I am confusing the system. I stopped buying airtime and still I am getting the same problem. I have refused to pay the account from jan 2016 until the problem is fixed. <br> <br> The call Centre keeps of fixing one month and then have the same problem next month, I have used more than R500 airtime trying to fix the problem with no luck