Standard bank check account overdaft

I have a cheque account with Standard bank and called them last year to make arrangements to pay the overdraft. Arrangement made is for R500 monthly and balance owing was R15000.00 as I had financial problem and to close that account as I will no longer utilise it. This month for the past two weeks I have received calls from them regarding this account. The balance is still the same and when I ask they tell me about the charges. I have already paid more than R3000 but can't get any explanation why thy charge me so much. I need to understand why the balance is still the same while I'm expecting it to be lesser. I can't continue paying the account for so long but there is no changes. I feel like I am treated unfairly by them. They even send me letters of final notice as if I'm not paying the account while they receive payment monthly from the date I made arrangements with them