Telkom double billing me

I have a phone contract with Telkom, however now I regret taking a phone contract with them. They send me wrong invoices and double bill me, when I check their site for my account history/ payment history, I noticed that they did not record my July payment, hence I now have a double billed amount. I had tried calling them before about this, as I keep getting calls from Nu Debt that I am in arrears whereas I am not,however when I called Telkom the agent said I am not in arrears, whereas this debt company keeps calling me and they keep sending me wrong invoices. For this month that say I will pay double than what I know I owe. I am tired of Telkom and everytime I call I don’t seem to be getting any help or my query being fixed.