Insurance company says they won't pay the surgery bill after all.

I was told before I agreed to the surgery that it was covered 100%.  Now they say my doctor isn't in network (he is; the insurance company gave me his name) and I didn't have a referral (I did; I even had my primary care doctor sign off on a medical exam for surgery; they wouldn't proceed without it.).  The insurance company wants a big chunk of money from me.  I'm on the phone now with them.