Tell them to send you a bill because you pay nothing without one. That has worked for my wife and I every time. Thats what insurance is for. Once they pay their part, then you can be billed for the rest. They cannot legally charge you for something that they do not know the amount of. My wife and I both worked in healthcare for over 35 years each. 
One thing that has caused most of this is the deals that hospitals have made with insurance companies. It is disgusting what hospitals, physicians, laboratories, imaging services, and most other departments get paid by insurance. A physician may charge the insurance company $250 for some procedure. The insurance company only reimburses the physician $8.50 because of the deal made between the insurance company and the hospital or physician group or whoever.  Or maybe they pay $3 for that $150 X-ray. That is why everyone in healthcare has to do 10 times the work they used to do because insurance pays almost nothing for services anymore.