Cell c bad service!!

I had a contract with cell c for my fibre line. Every month I made sure the amount for my debit order is available. One day my line just went off, so when. I phoned they told me I was behind on my account with R7100. I then checked my bank statements and there were months that they didn't take their money and then one month they tried to take double or even triple the amount. I emailed them and told them to drop my line. And that I would pays the amount back in installments. Yesterday they just took off the whole amount all at once. My rent money! This is unexceptable.it wasn't my fault that they didnt take the money each month. I made sure the amount was there every month.why didn't they just cut the line the first time. 10 months down the line they want to tell me I owe them that big amount. I feel it's their problem, not mine. As I was under the impression debit orders were taken off as I had internet connection the whole time😠😠😠