Disappointed client

I have been contacting discovery medical aid to understand the reason I got handed over to the debt collector. I had a discovery medical aid with my previous employer then had to cancel it when I got the new employer but I cancelled it to another discovery medical aid with now the new (current) employer. They told me that I have used some funds from the old one before it was cancelled then my first request was they must deduct from the current account the amount I used that didn’t happen. Then I had payment arrangement with them which they were deducting every month, this was 2024 mind you. I have never had any communication since we agreed on the payment arrangement. Now in march they sent a letter, who still sends letters now . To my old address and this letter was sent this year March telling me that I got handed over to the collectors without contacting me. Bare in mind that I am still using discovery and still paying every month how disappointing is that the did even called.