FWIW -- In the United States all medical billing is done according to what is known as the CPT coding system. This system began with the institution of the US Government's Medicare program back in the late 60's.
CPT stands for Current Procedural Terminology. This very complicated coding system was devised, and is owned and operated by, the American Medical Association.
Most day to day practicing Physicians are to some degree buffaloed by this system, so it is very easy for a code to be mis-classified either by the Physician, or somewhere else in the billing chain -- including of course, the person who does the actual coding for a bill. (One will never know where this has happened.)
It is quite a challenge to get this corrected once this happens -- not impossible -- but QUITE a challenge. When this happens one can expect that any correction will likely take several months. :(( I won't bore everyone with our long story in this regard ... and it was resolved after about 6 months of phone calls and repeated discussions. I finally demanded to meet the person responsible in the billing office in person -- which was when, and how, the case was resolved.

Best of luck with this situation,
Gayle