Billed for paid debt

I made payment in which I was overcharged, I have cancelled services with telkom and my account paid, they kept on debiting my account and sending me threatening messages but when I request a call back they do not call, when I call they say I must send proof of payment to telkom yet still threaten me with bureaus. On WhatsApp they said they are going to allocate my payment but still they say I haven't made payment. I am going to sue them