Microsoft 365 Error Codes

How to Fix Microsoft 365 Error 500121 on a New Phone

Emma Brown Sep 29, 2026 869 views

Question details

The user is unable to sign in to their Microsoft work account on a new phone due to authentication error code 500121.

How to Fix Microsoft 365 Error 500121 on a New Phone

Product: Microsoft 365
Device & OS: not provided
Scenario: Signing in to a work account on a newly purchased or replaced mobile device.
Observed behavior: The sign-in process fails during the multi-factor authentication (MFA) phase, displaying error code 500121 and blocking access to the work account.

Before you start

Ensure that you have access to a secondary authentication method if you set one up previously, such as a backup phone number or email, and verify that your new phone has a stable internet connection.

Solution 1Recommended

Use an Alternative Authentication Method or Contact Your IT Admin

The best approach for standard users whose primary MFA method is unavailable on the new phone.

Error 500121 generally means that multi-factor authentication was not completed successfully. Because your tokens do not automatically transfer to a new phone's authenticator app, you will need to bypass the prompt using an alternative method or have your admin reset your MFA configuration.

Select alternative method

On the sign-in screen, click on 'Sign in another way' or 'I can't use my Microsoft Authenticator app right now'.

Choose a verification option

Select another registered authentication method from the list, such as receiving a verification code via SMS or a backup email address.

Contact your administrator

If no alternative methods are available, contact your organization's IT administrator. Ask them to unblock your sign-in and require MFA re-registration in Microsoft Entra.

Use an Alternative Authentication Method or Contact Your IT Admin

For Administrators: Admins can easily reset the user's MFA settings in the Microsoft Entra Admin Center, which forces the user to register their new phone's authenticator app during their next login attempt.

Solution 2

Contact Microsoft Data Protection (For Sole Administrators)

Use this method if you are the only administrator for your organization and are locked out by Error 500121.

Solution 3

Clear Browser Data and Try Again

Sometimes corrupted cached data can cause authentication failures when trying to sign in via a web browser.

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Frequently Asked Questions

What exactly does Error 500121 mean in Microsoft 365?

Error 500121 indicates that multi-factor authentication (MFA) failed. This typically happens when the authentication request is denied, or the user is trying to sign in from a new device where the authenticator app hasn't been properly configured or transferred.

Why am I getting Error 500121 only on my new phone?

When you switch to a new phone, the Microsoft Authenticator app does not automatically transfer your secure security tokens. You need to re-register the new device or use a backup verification method like an SMS code to sign in.

Can I bypass Microsoft MFA myself to fix this error?

End users cannot bypass MFA if their organization strictly requires it. You must either use an alternative verification method registered to your account or contact your IT administrator to reset your MFA settings in Microsoft Entra.

Should I create a new tenant if I'm a locked-out administrator?

No. Do not create a new tenant unless specifically recommended by Microsoft Support as a recovery workaround, as doing so can cause significant billing, domain, and data management complications.

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