HARRASMENT regarding "unpaid" installment.

HORRIBLE service and no understanding. I have had a loan since 2015 with African Bank, I have never paid late and in fact have been making additional payments monthly to try and pay it off sooner. They constantly phone me for more loans which I decline. I requested for a payment gap (that was offered to me telephonically and via SMS) for end of January and end of February. This was approved. Now I am being phoned on a daily basis and threatened with the credit bureau. According to them I am behind on payments and they want "their money". I received my statement last week CLEARY indicating that I am still ahead (advance, in credit) with R11327.34 (money I paid to much - over and above the monthly instalment). According to the bank that means nothing I still need to pay the money that I got a payment gap for.