Unfair treatment and poor administration

I was hospitalized in September 2018 and received one account from NHLS in November 2018 which I paid immediately. Suddenly in 2019 I started receiving threatening SMS's from a place called Credit Intel about outstanding payments due to NHLS demanding immediate payments. When I phoned them, they gave me an email address and I forwarded my proof of payment to them. After having to follow-up on them after receiving yet more SMS's, I received an email informing me that there was actually a second invoice. I never received that invoice, but regardless went and paid that amount immediately and also emailed proof of payment. Next moment I received an invoice from Credit Intel billing me for the fact that NHLS handed me over to them for the full amount of both invoices and for all sort of admin costs from their side, including for each SMS they've sent me. I responded to that email and again attached my proof of payment and never even received the decency of a response, despite receiving a "read receipt" confirmation that my email was read. To this day, I keep getting threatening SMS's where the amount just keeps increasing, demanding me to pay in money that I've already paid in. This is ridiculous and unfair treatment and no one dares to respond. I'll be sending a follow-up on my complaint email to every email address I could find on the internet today and hopefully someone will take ownership and sort out their poor administration.