Never give your Medicare number to someone who calls or visits
Added
Scam calls pick up around Medicare Open Enrollment. Medicare won't call to sell you anything or visit your home, so hang up and keep your Medicare and Social Security numbers to yourself. Don't join a plan over the phone unless you made the call, and check your Medicare statements for services you didn't get.
The result
Result: Medicare.gov: suspected fraud can be reported at 1-800-MEDICARE (1-800-633-4227) or online, and a Medicare.gov account lets you see Original Medicare claims as soon as they're processed.
Source
Medicare.gov: Reporting Medicare fraud & abuse (opens the original source in a new tab). Check it for the latest numbers before you act.