I have never had to use the ambulance. I have been to the hospital but always went with a friend.
From those who I know have used a private ambulance, they paid by credit card and got reimbursed by their health insurance company back home. I didnt want to pry into their private matters to find out how much they paid or how much they got reimbursed, it didnt seem like any business of mine.
Feel free to contact the companies yourself if you so desire.