Nimble Group are *********** clients

My account was sold to Nimble Group and I received a call to from Nimble Group confirming that the account was sold to them and I requested the statement on the 22nd June and I received it on the 23rd June and it showed that I owe them R3 771.01 which I made it in full on the 25th June and asked for the settlement letter but no response.

Today I asked the statement again to confirm with myself that the balance has been settled however I see that there is a fee of R433.77 that has been added from the Zero Balance outstanding to R433.77.. To my surprise I see the was a fee added on the previous months of R23 for the last two months but now it jumped from R23 to R433.77

I called them to understand why was the fee added after I have settled the outstanding balance and the people there keep on canceling my calls when I asked such questions