Handed over to Debt collectors - WHY?

After contacting my medical aid re: my burst appendix, I made arrangements with the Credit controller of Linmed to pay the outstanding amount (my share) after the gross amount was paid by my medical aid. From 15/8/2016 I've paid R500 every month into their account and I never missed one payment. After 15 June 2017 I tried to contacted Nicolene Ehlers from Linmed wanting an invoice to check the outstanding amount. Mrs Ehlers unfortunately resigned. Then after contacting Netcare customer care, Trudie emailed me my statement on 26 June 2017 saying I must use Case nr ..... (Eyes only) (Which I've been doing this hole time) In the meantime I received a phone call from a dept collector company saying I must pay them and not Linmed because there is no arrangement in place with Linmed. I've emailed Ronel van Niekerk, Geraldine Roux and Annemarie Berry on Mon 2017/08/21 09:36 AM with this query. To date I still do not know why Linmed handed me over even though I made an arrangement with them. What is going on?