National Health Laboratory Service account already paid

I went for my annual hospital check up and had bloods taken 18 Aug 2017. Then I proactively called NHLS to ask them to submit the claim to my medical aid. It was eventually done and was paid by my medical aid 31 Oct 2017. The NHLS then sent a final order letter to my previous address demanding payment in November 2017 followed by threatening sms. I called and emailed I spoke to Jerry Rikhotso and emailed Sithembiso Henry Nyandeni Jan 2018 asking then to please allocate the payment to my account. I send them proof of payment 27 Feb 2018 and I included accountqueries email when I sent them. 02 Mar 2018 I did another follow up and received no response. Now I'm receiving calls from debt collectors over and payment that has been paid 5 months ago. I'm am extremely frustrated that no one is doing there job in that accounts department at the National Health Laboratory Service and when I call I have to hold for 10 mins before I get through only to be told that the person is on leave. I want this matter resolved.