You can enroll through the insurance company's website or by mailing them an enrollment form. You can also enroll through Medicare's website or by calling 1-800-MEDICARE (1-800-633-4227). There are certain times you can enroll without a late penalty, including your seven-month Initial Enrollment period, Open Enrollment between October 15 and December 7, and the seven-month period around your 25th month of disability.