Medical Insurance Fraud?

I was referred to a dentist by my doctor for a sleep apnea oral appliance. I called that dentist and asked if they take my insurance.

They told me yes over the phone. In the office they tell me I owe 1250 deductible because they are out of network.

I have since educated myself that while deceptive, they technically did not lie to me.

Why i suspect there is Fraud involved… I received a letter from my insurance company from a different, unaffiliated doctor approving the appliance in full. Different town different doctor and the treatment is not only covered but approved.

Am I missing something? Is this common practice?

It says approved, is a billing determination letter and was clearly submitted by this unknown doctor in network. Thoughts?