There are many so many scenarios where Medicare exists along with other health plans that it is impossible to give a blanket answer to this question. But there are some very good publications put out by Medicare that spell out each situation where Medicare is and is not your primary insurance. You can find this information by going to medicare.gov.
Also, you can call COBC (Medicare Coordination of Benefits Contractor) at 1-800-999-1188.
Also, you can call COBC (Medicare Coordination of Benefits Contractor) at 1-800-999-1188.