I've never dealt with this situation before, but I would suggest that you print copies of the materials from the insurance company's website where they list the place as an in-network provider. Also, put together a set of notes to document the time and date of the phone call you made to them.
This information might come in handy in the event of a billing dispute. If the insurance company continues to give you a hard time, you should look to pursue this through your state insurance commission.