Medicare claim number which is also known as Medicare Beneficiary Identifier (MBI) is the number that Medicare uses to file your claims. MBI are numbers and upper-case letters that you may find on  your Medicare card. If you enrolled in Medicare prior to 2018, you should have received a new Medicare card. The MBI used to be your social security number with a letter after it. Between April 2018 and April 2019 the CMS (Centers for Medicare & Medicare Services which is governmental agency that administers Medicare) mailed new Medicare cards which had a new MBI instead of your social security number to prevent identity theft and tax payer fraud.