Most of the things your doctor recommends will be
covered by your plan, but some may not. When you have a test or
treatment that isn't covered, or you get a prescription filled for a
drug that isn't covered, your insurance company won't pay the bill.
This is often called "denying the claim." You can still obtain the
treatment your doctor recommended, but you will have to pay for it
yourself.
If your insurance company denies your claim, you have the right to
appeal (challenge) the decision. Before you decide to appeal, know
your insurance company's appeal process. This should be discussed in
your plan handbook. Also, ask your doctor for his or her opinion. If
your doctor thinks it's right to make an appeal, he or she may be
able to help you through the process.
Source
American Academy of Family Physicians