They don't know what they're doing

In June 2020 HP Attorneys contacted me to put a payment plan in place for an outstanding debt I had with a cellphone company. In March this year, I requested a settlement amount from them via their WhatsApp group, once received I paid immediately and requested a settlement letter (which I received after many phone calls and e-mails back and forth), after receiving the settlement confirmation letter, I got a phone call from one of HP Attorney's employees telling me I have just over R 500 outstanding for legal fees (payable to HP Attorneys). My husband called them to query and was told that there were no fees outstanding because we had received a settlement confirmation letter. I have now received a call following up on the payment of the supposed outstanding legal fees. My question is, why was the settlement amount inclusive of all fees and why should I pay this now if I have a letter stating the debt is now paid up