Medicross Carlswald handed me over without notice

I took my daughter for consultation on 17th May 2022. The medial aid couldn't cover I have to pay out of my pocket. Then when the invoice was sent to me I realized there was a desc****ncy the invoice had listed that I also took my son as well which is correct. I then emailed them back to fix it for me to make a payment. 3 days later I receive a call from a debt collector without prior notice I didn't refuse to pay just wanted the invoice to be fix. How can they hand over the account without notice. And I also wonder if the medical aid was paying they would have double charged. I'm really not happy and don't think I will ever go there again