Nu debt Management Sucks

I am frustrated with this debt collector. my MTN account was handed over to them after I missed a few payments. I have since paid off the account in full, including all collection fees. Despite this, I am being told that I still owe money because a payment I made in March has not been updated in by their finance department. I sent proof of this payment in April.

I settled the account over a month ago, but each time I call, I am incorrectly informed that I still have an outstanding balance. Furthermore, when I requested a statement, the payment from March is not reflecting.