Extra charges on account statement

I took out a laptop contract in 2016. My last payment was to be 25/02/2020. My February statement 2020 reflects last payment but there is a outstanding amount of more than R5000.00. I have called and emailed fonechoice I am not receiving any feedback on the emails sent. The call centre agents all say that they can not assist me and no one can explain were the outstanding R5000.00 on the account comes from. I spoke to a lady in February and she advised that this was a system error and that they have received other calls from customers with the same problem. This is not being resolved I am not getting any feedback from them and they continued to deduct the monthly payment from my account.