Billing and NPs

If an NP bills under a physician's number, does this mean that the physician supervised them on that office visit? Can a NP bill under a physician's number without that physician being within vicinity?

In my area no. The Physician does not have to be present. We have to branches to our office and on one we only have a doctor there on Mondays, tuesdays, and wednesday mornings (FNP's by themselves the rest of the week). But they can still bill out.

Then I guess my question is that if physicians are not supervising the NP but the NP is using the physician's number to bill with isn't that a problem that could lead to chart audits? It would seem to me that the NP would bill under their own number if they don't require supervision from the physician. I don't mean to be difficult but what is the benefit of billing under the physician's number if they are not supervising the NP?