How can I add my phone number to my existing 365 Family account?

To add a phone number and sort out the account confusion, use the Microsoft account pages rather than device links.

1. Add a phone number (alias) to the Microsoft account
   1. Go to the “how you sign in” page: https://account.live.com/names/manage (this opens the Your info / alias management page).
   2. Sign in with the Microsoft account that has the 365 Family subscription.
   3. Select Add phone number.
   4. Follow the prompts to add the mobile number as an alias.
      • VOIP numbers cannot be used.
      • If the number is already in use on another Microsoft account, you will see a message like “phone number is already a Microsoft account” or “phone number is already used to sign in”. In that case:
        • Choose Sign in if it belongs to an account owned and keep using it there, or
        • Choose Transfer number to move it to the Family account, and follow the prompts.
        • If Transfer number does not appear, the number was recently used; select Change number or try again after 30 days.
2. Confirm and manage aliases
   1. On the same page (https://account.live.com/names/manage), review all email addresses and phone numbers listed.
   2. Use Add email, Add phone number, or Remove next to entries that should not be associated.
   3. Any alias listed there can be used to sign in; all share the same password and account data.
3. Fix the “no phone number” / text-code issues
   • Microsoft sends text messages only to the primary phone number on the account. If no mobile number is set, verification will fall back to email.
   • After adding the phone number as above, go to the security page (https://account.microsoft.com/security) and verify the number appears under Security info / recovery options.
   • If text codes still do not arrive, remove the phone entry under Security info, then add it again and complete verification.
4. Handle the second (personal) Microsoft account
   • If the phone number or an email address is tied to a different personal Microsoft account that cannot be accessed, use the standard account recovery/sign-in help starting from the sign-in page and the “Forgot password” or “Can’t access your account” options.
   • If the phone number is attached to that other account and needs to be moved, use the Transfer number flow described above when prompted that the phone is already in use.
5. Reaching support without getting blocked by phone-number checks
   • For Microsoft account issues (including billing/autopay), start from the general Microsoft account support entry point and use the Get Help option rather than relying on automated phone routing.
   • If the subscription and autopay are on a personal Microsoft account that cannot be accessed, resolving sign-in to that account (via recovery and alias/phone management) is required before the payment method or subscription can be changed.

References:

• How to add an email address or phone number to your Microsoft account
• Help with "phone number is already used" in a Microsoft account
• Change the email address or phone number for your Microsoft account
• Why is Microsoft texting me?
• What can I change on my Microsoft account?
• I need a phone number for Microsoft support - Microsoft Q&A