GL Attorneys - Cannot understand basic math and not Professionalism at all

Being a Single Mother and only income, I made an arrangement to pay back an FNB Loan account at R50 a month with oustanding amount of R2900. After accidentally transferring money from my Bank Account to the Loan Account of R500 which ended up being short of the month and that in a Christmas period, I emailed GL Attorneys the error that occurred and said the next 10 months is paid up front now. After numerous of phone calls everyday, demand letters they still don't understand the basic math of paying 10 months up front. I called the call centre to explain this for the thousands of time and whenever GL Attorneys don't accept my answer they insist to still receive R50 monthly. Even worse, if I ask to speak to the Supervisor the phone gets put down in my ear. I am happy to pay my R50 arrangement with them but the rude service and not understanding basic Math is disgusting that they treat people the way they do. GL Attorneys you should be ashamed of yourself!