Who should the health insurance professional contact when there is a billing question?

It is not clear what you mean by a "health insurance professional". If you are referring to an insurance billing person an a provider's office, that person should contact the insurer to determine whether, and to what extent, a procedure is covered.

If you are referring to an insurance agent who has a question about a premium statement sent to a client, likewise, the insurance company should be contacted.

There may be other types of contacts that should be made, so feel free to further clarify the question.