Vodacom contract

I have a new contract when my first debetorder whent through vodacom managed to try and take the installment from a bank account that has been closed for over a year now. I phoned to find out why my installment didn't go of my account. Thats when we fond the problem in regards to them using the wrong bank account. Thy then rectified this by changing my account details on there system to my fnb acc. I then whent to house and home and did a cash payment for the installment thy wrongfully did (i still whant to know how thy got my old std banking details as afordability and aplication was done using my fnb acc.) now thy are charging me R100. 88 penalty fees for there mistake htf am i liable for there fup