US Medicare Provider and Supplier Enrollment (42 CFR Part 424 Subpart P) 424.505-506: Be enrolled in Medicare before billing Medicare or a beneficiary for covered items or services, report your NPI on the enrollment application and include the NPIs of all identified providers on every claim

To receive payment for covered Medicare items or services, from Medicare on an assigned claim or from a beneficiary on an unassigned claim, a provider or supplier must be enrolled in the Medicare program; once enrolled it receives billing privileges and a billing number effective for the date of service. A provider or supplier eligible for a National Provider Identifier must report its NPI on its enrollment application (and update its record with the NPI if it enrolled before obtaining one), must include its own NPI and the NPIs of any other providers or suppliers identified on every paper or electronic claim, and a contractor rejects claims without the required NPIs. A physician or eligible professional who has validly opted out need not enroll for any purpose, including to order or certify.