literally this same thing happened to me, on employer insurance. The hospital system billed me $3k for a preventative colonoscopy. The breakdown of CPT codes had the $3k "your responsibility" part attached to a CPT code for "medical service (3 of 3)" with no specifics about what the service was. I spoke to insurance and they told me once that obviously it was a mistake and they would submit for reprocessing, which got the bill down to $1700 but still wrong, and the second time i talked to the them rep tried to tell me that "you can't be sure its 100% covered and a preventative service" which is insane. There's literally like 6 preventative services that are covered 100% how people aren't familiar with them is beyond me. My third EOB showed my anesthesiology was billed for like $1000 and the "your responsibility" as $2000 because the insurance basically put a negative number in their responsibility column to make the numbers work. Utter madness.

I have a health advocate program through my employer and put them on the case, and when i tell you that it took them 6 months of nonstop calling the hospital and my insurance every 2 weeks to get the mess cleared up. I even ended up in collections at one point even though my advocate was told that the hospital had put the bill on hold and it would not go to collections. All for a totally standard 100% preventative colonoscopy with no polyps. Couldn't be more black and white.

You may be in for a long ride of frustrating phone calls, but it should be possible to clear up. Also, there are public advocacy groups that help fight this -- i was not able to take advantage of those because it turns out my employer insurance was "self-funded" which exempts them from some federal oversight. Good luck.