This just happened to me recently and I'd like to share this in the hopes that it would help others that might run into this situation.
TL;DR/Moral of the story: always double-check whether your doctor's billing department bills directly to insurance or does it through a convoluted intermediary.
I went to a PT office in Aug and got a splint done for my wrist. I double-checked with the front desk to verify my insurance coverage and paid the deductible up front. A few months go by and I get a bill all of a sudden for the remaining balance of the splint. I called the office to inquire and they informed me that they actually use a third party biller and provided me with their phone number.
I called the biller and was told that they billed my insurance for the splint but the claim was denied and as a matter of fact, my insurance does not cover splints. Now even though I had an HDHP, it was a group insurance plan through my employer so I knew we had pretty good benefits and I would at least get a discounted price if nothing else (vs the full price I was billed). So I called my insurance to see why they would deny the claim. The rep told me that they did not in fact, receive a claim at all from the biller so there is no way they could have denied it.
At this point, I was pretty frustrated so I called the biller again and told them that my insurance didn't even receive a claim. The billing office was adamant that they billed the splint and even gave me a claim number. So I ended up calling the insurance yet again to give them the claim number. Not surprisingly they weren't able to find any claims associated with that number. I asked them how that may because I went to that office a few times after I got the splint for physical therapy and every time the services for my PT were billed and processed by my insurance. The rep did some digging and found out that the biller from the PT office actually bills all services to an intermediary healthcare service that in turn have a contract with and bills my insurance for anything PT related. BUT here's the kicker: they only bill PT related services but not prosthetics/medical equipment like splints. At this point, I realized that the biller for the office billed the splint to the intermediary instead of billing my insurance directly. Now I want to give the biller the benefit of the doubt but they have been around for a while so there is no way they could not have known the claim would have being rejected. I'm guessing they billed the intermediary on purpose knowing it would get rejected so they can bill me directly for the full amount because my insurance "rejected" the "claim".
In the end, the insurance rep transferred me to another department that called the biller directly on my behalf and not a day later I see a claim for a splint that had a much-discounted rate than what I was billed for.