TFG is asking me to pay for their error

I paid late in December which caused my account to be in arrears, I called in to make an arrangement. I was told to pay R2135 for four concecutive months and the matter will be resolved. I paid the first month, I got a call weeks later, I told the debt collector I made an arrangement. She told me you can only make an arrangement for 1 month, the next month you must do another arrangement, if people call you now ignore them. I think they charge you everytime they call, which sounds like ****ming people if the arrangement has to be re-done every month.

Two months in I notice there is no change in the amount I'm requested to pay to make things right. I called and was told the lady at the Markhams counter captured the payment wrong, she opened a 12 month account instead of putting the money in the 6 month account that I have. Now the money gets split into the two accounts that's why it's not making a difference when I pay. The gentleman said he will fix it, I asked for that in writing he said they don't do that. I must pay at the end of the month and it will show up on the slip if they fixed it. I have been called twice already, noone is acknowledging the error, they are still calling me to pay the large amount which is the result of their person's error. How is this fair?