Unacceptable blacklisting of clients

I was previously admitted in this hospital and I had a co-payment. I received a call from the credit officer Regina Tshehle regarding this co-payment and an invoice was forwarded to me. I started paying this account from January and in March I received a call from the debt collectors regarding this account and I informed them that I have been paying. I called Regina to get clarity and she said the payments were reflecting and she was to send communication to the debt collectors. I accepted that as errors do happen. I recently received a call again from the BL Debt Collectors regarding this account which was paid up in March and was told that I'm blacklisted. I sent an email to Regina on 30/04/19 requesting clarity regarding the R599.72 that I am said to be owing and I did not receive any feedback to date. I am interested to know what is the process before handing over an account and for blacklisting. She was made aware the first time that she did not check the account properly as the payment was made, now why did she not call to find out if payment was made and when before tarnishing my credit record?