Worst service provider

I took my first ever contract with Telekom mobile internet services last year and it's been the worst experience of my life. Their customer service department is very useless, they do very little or nothing to help you. My account was debited twice because I made a late payment. My initial installment is R1100 per month and because of late payment I went online and paid R1300 which included late payment fee. My services were restored but a week later they debited R1800 from my account which does not make sense and the agent couldn't give me a proper explanation as to how they got to that calculation, she says it's payment for the current month and next month which does not add up, my question is, why take payment in advance for next month because I did not give them authority to do that. I request a reversal as that was the last money in my account to keep me going until month end and they say they cannot reverse because the account was handed to collections and it will take 7 days for the money to reflect on their end so they can reverse it. It's not the first time it happens, they take money from your account as and when they please. I have canceled the contract and waiting for the final invoice. I can't afford this headache.