I actually just paid one of these myself. But, I think they have to at least inform you or give you some notice. I dont see how they can charge you if you did not know about it.
If you have health insurance you might consider disputing the claim for payment form the doctor's office.
I agree that in order for the office to have a valid claim, you should be on notice of the policy. If the appointment was made over the phone it is likely you can dispute any charges with the office, or come to a settlement on a lower fee.