ProntoFax is now in the Claude directory. Fax from a chat.

EN ES
Directory Send a fax

All notes

How to fax your Medicare Part B form (CMS-40B) to Social Security

If you already have Medicare Part A and want to add Part B, the paper form is the CMS-40B. The form itself says to mail or fax it, signed, to your local Social Security office. Here is what goes in the fax, where it goes and how to send it from your phone.

Who this is for

You have Part A and you are adding Part B. That usually means one of three situations:

  1. You are turning 65. Your Initial Enrollment Period is 7 months long. It starts 3 months before the month you turn 65 and ends 3 months after it.
  2. You are leaving job-based coverage. If you or your spouse had health insurance through a current job, you get a Special Enrollment Period. It lasts up to 8 months after the job or the coverage ends, whichever comes first. COBRA does not extend it.
  3. You missed both. The General Enrollment Period runs January 1 to March 31 every year, and coverage starts the month after you sign up. You may owe a late enrollment penalty.

Medicare open enrollment, October 15 to December 7, is a different thing. That window is for changing Medicare Advantage and drug plans, and it does not involve this form.

What you are sending

  1. The CMS-40B, signed. The current version is dated 07/2025 and runs 3 pages: an instruction page and two pages to fill in. It asks for your Medicare number, whether you had employer coverage since you turned 65, and your signature.
  2. Your coverage start date, on the form. The 07/2025 form has a “Choose your coverage start date” box. If you are still covered by a job-based plan, or in the first full month after it ends, you can pick the first day of the month you enroll or the first day of any of the next 3 months. Older instructions tell you to write the start month in a remarks section. The current form has no remarks section, so use the box.
  3. The CMS-L564, if you are using job-based coverage to qualify. It is 2 pages. You fill in Section A and your employer fills in and signs Section B. Fax it together with the CMS-40B.
  4. Proof of coverage, if your employer cannot fill in the L564. Fill in Section B yourself as best you can and leave it unsigned. Then add proof of your job-based insurance. Medicare lists these kinds of proof:
    • An income tax form that shows health insurance premiums paid
    • W-2s showing pre-tax medical contributions
    • Pay stubs showing health insurance premium deductions
    • Health insurance cards with a policy effective date
    • Explanations of benefits paid by the employer plan
    • Statements or receipts that show premium payments
  5. Proof for question 4, if it applies. The CMS-40B asks whether an employer, insurer or anyone else asked or required you to sign up for Part B. If yes, explain why on the form and attach the documents.

If you missed your chance to sign up for a reason outside these cases, such as a natural disaster or an emergency, Social Security uses a different form for that Special Enrollment Period, the CMS-10797. Its Part B sign-up page explains when it applies.

In practice the whole fax is the 3-page CMS-40B, the 2-page L564 and a few pages of proof. That is well under the 25-page limit for one fax.

Where it goes

Both forms give one destination: your local Social Security office. That is the only place Social Security tells you to send them. It does not publish a separate national fax number for these forms, so there is nothing else to look up.

  1. Find your office with the Social Security office locator. Enter your ZIP code and open the office’s details. Its contact numbers list a phone, a fax and a TTY number.
  2. Check it against our Social Security fax numbers by state. Pick your state to see the fax number Social Security published for each office, with the date we read it. You can also search the fax number directory by city.
  3. If the office shows no fax number, call the office, or call Social Security at 1-800-772-1213 (TTY 1-800-325-0778). Do not use a number from a forum or an old blog post.
  4. Read the number back digit by digit before you send.

The cover page

A cover page helps the office match your fax to you. Put this on it:

  • Your full name as it appears on your Medicare card
  • A daytime phone number
  • “Medicare Part B enrollment, CMS-40B and CMS-L564 enclosed” (leave off the L564 if you are not sending it)
  • The total number of pages, including the cover page

Leave your Social Security number off the cover page. The L564 already asks for it, and the CMS-40B carries your Medicare number. The cover page is the one sheet that does not need either.

Send it from your phone

  1. Open ProntoFax in your phone’s browser. Take a photo of each page or upload a PDF, and add the cover page.
  2. Enter your office’s fax number. We check that the line can receive a fax before you pay.
  3. Pay $2.99. No account. You are charged only if the fax is delivered.

If you want to see what the office will receive before you send the real one, a test fax sends your pages to a ProntoFax test line and shows you what arrived.

After you send

Keep the confirmation email. It shows the delivery time and the page count, with a link to view what you sent.

Social Security mails you a new Medicare card once your Part B is processed. If nothing arrives, call your office and give them the date and time from your confirmation email. A delivered fax means the pages reached the office’s machine. It does not mean the office has processed them, so keep your originals.

This note is about getting your forms where they need to go. It is not advice about which enrollment period you qualify for. If you are unsure, your State Health Insurance Assistance Program offers free counseling.

Sources

Send it from your phone

$2.99 flat per fax, up to 25 pages, no account. You are charged only when delivery is confirmed.

Send a fax

Or send a test fax first