Harassment after account paid up

Last year I made an agreement to pay off the amount I owed via debit order. The amount I was told that I had outstanding was R4721. The agreement I made was R500 every month until the account was done. I started my first payment of R500 at the end of July 2018 and my last debit order was R342 in April 2019. If you calculate from July to March, that is R4500 plus the R342 at the end of April will give you an amount paid of R4842. After my final payment I received no contact until June when I received an sms saying I still owed them. I tried countless times to get a manager or supervisor to contact me but I received no call of such. The only call I kept receiving was from call Centre agents who just kept telling me I owed them. Every time I get harassed with calls and sms but not a single manager contacts me. I even sent WhatsApp asking this matter to be resolved as according to what I paid my account should be closed but all I get is harassment from agents and sms.