Failed to debit my account for 3months. Now they say, im in arrears

Tracker deliberately failed to debit my account for 3months. There was always money in my account and now im being told i owe R1117. This is absurd and I will not pay them a cent. I have never received a call from them to attempt to debit my account. This is not the first time this has happened. So now, they have suspended my service thank God as im now with another provider. I have never seen such poor service. They sent an sms yesterday saying they will reduced the amount to 558. Who's fault is it that they didn't attempt to debit my account Bunch if ******