The Centers for Medicare & Medicaid Services has issued an FAQ that explains about the use of a non-NPI for prescribers who are not covered entities.
Payers can work with pharmacies to come up with a non-national provider identifier or default alternative on billing transactions for physicians who do not use an NPI. In an FAQ #9100, the Centers for Medicare & Medicaid Services explained that some physicians are not covered entities and do not require an NPI. In such cases, a pharmacy can use a non-NPI prescriber ID if allowed by the payer. However, CMS cautioned that this default should be used only in rare cases.