Question: how to sign up for medicare part b if you already have part a?
Answer: Call Social Security at 1-800-772-1213 (TTY: 1-800-325-0778). If you worked for a railroad, call the RRB at 1-877-772-5772. If you already have Part A and want to sign up for Part B, complete an Application for Enrollment in Part B (CMS-40B).