I cannot add a phone number to my account information.

If the phone number cannot be added to account information, the most common causes are:

1. The number is already linked to another Microsoft account
   • A phone number can only be added to one Microsoft account.
   • If the error says the number has already been taken or is already used to sign in, sign in to the other Microsoft account that currently has the number, remove it, sign out, then add it to the intended account.
   • If a Transfer option appears, use that.
   • If Transfer does not appear, the number was recently used on another account. In that case, use a different mobile number and try again after 30 days.
2. The number may be a recycled number
   • Mobile carriers can reuse numbers after 30 days.
   • If the previous owner linked that number to a Microsoft account recently, the only current option is to use a different mobile number.
3. The number must be a mobile number, not VOIP
   • VOIP numbers cannot be added as a way to sign in or receive verification codes.
4. Phone number sign-in options may be limited
   • The option to add a new phone number may not be available for all Microsoft accounts.
   • Some accounts may only show options such as a recovery email address, Microsoft Authenticator, or a passkey.
5. If the issue is specific to Vietnam
   • If the page shows that the country code is not supported, the accepted guidance is that if the country is not listed in the drop-down, that number cannot currently be added.
   • In that case, use another recovery method such as a recovery email, Microsoft Authenticator, or a passkey.

A practical path to resolve it:

1. Go to the Microsoft account Security page: https://account.microsoft.com/security
2. Select Manage how I sign in.
3. Check whether Add a new way to sign in or verify is available.
4. If phone is available, add the mobile number there.
5. If phone is not available, add a recovery email, Microsoft Authenticator, or a passkey instead.
6. If the error says the number is already in use, remove it from the other Microsoft account first, or use Transfer if offered.
7. If the number was recently used on another account, wait 30 days or use a different mobile number.

Also avoid making excessive repeated verification requests, because that can temporarily block code delivery.

References:

• Help with reusing phone numbers in a Microsoft account
• Microsoft account security info & verification codes
• Troubleshoot Microsoft verification code issues
• I can't add my phone number country code - Microsoft Q&A