Authenticator not letting me login without using old phone number

Use these steps, depending on what is actually blocking sign-in:

1. If the issue is MFA on a Microsoft 365 / Entra ID account
   • Go to the Additional security verification / Security info page for the account (from a browser where the account can still sign in, such as a PC that is already logged in).
   • Remove the old phone from the MFA methods.
     • On the Security info page, delete the entry for the old phone number.
   • Add the new phone:
     • Choose Add sign-in method → Phone, then enter the new phone number and complete verification.
   • Optionally, install Microsoft Authenticator on the new phone and add it as an additional method following the prompts.
2. If push notifications are going to the old phone
   • A sign-in approval might still be sent to the Authenticator app on the old device.
   • If the old device is available, open Authenticator there and approve once, then:
     • Remove old devices from the account security settings.
     • Re-register Authenticator on the new phone.
   • If the old device is not available, use another verification method (such as SMS) if offered, then clean up old devices and methods in the account security settings.
3. If locked out of a business (work/school) account and no alternative method works
   • An admin for the tenant must reset MFA:
     • In Microsoft Entra Admin Center → Users → select the user → Authentication methods → Require re-register multifactor authentication.
   • If the friend is the only global admin and is locked out, a support ticket with Microsoft (Data Protection team) is required, initiated via Microsoft support phone or via another tenant as described in the Q&A guidance.
4. If locked out of a personal Microsoft account and no method works
   • Use any device where the account is still signed in (for example, a Windows PC or browser session) to:
     • Turn off 2FA temporarily on the account security page.
     • Update the phone number and reconfigure MFA/Authenticator with the new phone.

Key idea: use an existing signed-in session or an admin to remove the old phone as an MFA method, add the new phone number and/or re-register Microsoft Authenticator, and clean up old devices so notifications and codes no longer depend on the old number.

References:

• Use a screen reader to set up and troubleshoot multifactor authentication
• Microsoft Authenticator FAQs
• I need to reset my mfa methods - Microsoft Q&A
• I have a Micrsoft business account and I am the admin and the only user registered under this account. I can't log in because i have replaced my phone and the authenticator doesn't work. - Microsoft Q&A
• Microsoft authenticator sending me into a spiral - Microsoft Q&A
• Severity A Situation - Locked out of global admin account (MFA) - business down - Microsoft Q&A
• Cannot authenticate with E-mail when logging into Microsoft Authenticator(iOS). - Microsoft Q&A