I recently dealt with this when I went to a new doctor and rather than code the visit as annual/preventive (included at zero cost) they billed it as "new patient" and charged me hundreds of dollars. I called their billing and challenged it, especially since we had 5 visits within a week (one each for the 5 members of our family), and they properly coded my two young kids as preventive but not my husband, myself, or my 18 year old. I haven't actually gotten a response back, and they did say they would call back within ten days, but if I go to my HSA account the claims are now shown as $0 instead of unpaid with a balance, so I guess it's been resolved.
Good luck.