Fees which are not my responsibility to pay

I have been trying to get ahold of Nthabiseng Selepe or Mandy Purves from Ceebee Debt Management Services since last week, and it seems to me that they are no longer taking my calls. Ceebee was enlisted by my son's school to collect outstanding school fees. The issue however is that I paid my sons fees in full on 20 December 2021. Prior to this I received no communication of my sons account being handed over to debt collectors due to non payment of school fees. In March (4 months after my son's fees were fully paid) I received and email saying I had outstanding fees. I responded to Nthabiseng's email and also attached all the POP's and explaining what happened. She did not respond or get back to me even after I had sent follow up emails. I then received another email beginning September saying I must pay R517.92 for the expenses and fees incurred. This is not my problem and my sons school needs to pay this amount as I did not enlist their services. Since the communicating with Mandy last I have had n feedback even after she promised to call me back. I will not let this rest until this matter is resolved.