New Medicare Card Transition Ends Next Week: Claim Reject Codes Beginning January 1
If you want to get paid you should be reporting MBIs on all of your Medicare claims. The deadline is here: if you are not using Medicare Beneficiary Identifiers (MBIs) on claims (with a few exceptions) after January 1, regardless of the date of service, here are the denials you will receive:
Electronic claims reject codes: Claims Status Category Code of A7 (acknowledgment rejected for invalid information), a Claims Status Code of 164 (entity’s contract/member number), and an Entity Code of IL (subscriber)
Paper claims notices: Claim Adjustment Reason Code (CARC) 16 “Claim/service lacks information or has submission/billing error(s)” and Remittance Advice Remark Code (RARC) N382 “Missing/incomplete/invalid patient identifier”
Read more about it here, MLN SE18006 New Medicare Beneficiary Identifier (MBI) Get It, Use It

References:
CMS.gov