An old land line phone number which no longer exists.

For a personal Microsoft account, these are the supported options:

1. At the sign-in prompt, check whether another verification method is available:
   • Select Show more verification methods if it appears.
   • If another email, Authenticator, or other method is listed, use that instead of the old landline.
2. If no usable method is available, select I don't have any of these during sign-in.
   • This starts the process to replace security info.
   • When all security info must be replaced, there is a 30-day wait before sign-in is restored.
   • That wait cannot be bypassed.
3. Do not try to reuse the mobile number that is already attached to another Microsoft account.
   • A phone number can only be added to one Microsoft account.
   • If the mobile number is already linked to the secondary account, it cannot be added to the primary account unless a different number is used.
   • If the number was only recently added to the other account, the only option is to use a different mobile number.
4. If access still exists to the secondary account, add a different sign-in method there first, then remove the mobile number from that account.
   • Microsoft accounts can have up to 10 ways to verify sign-in.
   • The option to add a new phone number may not always be available; other methods can include a recovery email, Microsoft Authenticator, or a passkey.
   • Avoid changing all security info at the same time, because that can trigger another 30-day restriction.
5. If verification codes are not arriving or sign-in is blocked, avoid repeated attempts.
   • Excessive or repetitive requests can cause temporary blocks.
   • If blocked, try another verification method, try another network, and wait before trying again.
6. Microsoft Support cannot send verification codes or change account details for this scenario.

If the primary account is already in the 30-day pending period and no other verification method is available, there is no faster self-service path in the documented process. The practical next step is to wait for the security info replacement to complete, or regain access to any existing verification method if possible.

• Troubleshoot Microsoft verification code issues | Microsoft Support
• Help with reusing phone numbers in a Microsoft account | Microsoft Support
• Microsoft account security info & verification codes | Microsoft Support
• How can I change my security landline number to a cellphone number that can receive a code? - Microsoft Q&A
• MFA code being sent to old phone number - Microsoft Q&A