Glynwood debt harasment

I was a patient at Glynwood on the 18th of September 2020. I was taken by ambulance to the casualty ward were they treated me and then send me home. I paid everything cash, but then they charged me for being there the 19th as well for the sam thing that I paid for on the 18th. I phoned and spoke too Lucas who said I must come in, I did and they told me that they will fix it. But I was handed over too a debt coleccting comp. I send emails too Glynwood account and too the collecting comp. But till now no one bothered too contact me back. I just keep on getting this threatening sms from the debt companny. I don't owe them a send. What can I do for them to stop this harasment and putting my credit name to shame?