Hi @Karsten Riegler,
You'll need the Microsoft 365 admins in your organization to reset the account so a new QR code can be generated with which you can relink the authentication device to your account on your new iPhone.
The steps to perform with your admin help,
- Go to Azure portal >> Microsoft Entra ID >> Users >> Click "Display name" of the issue account.
- Click "MFA status".
- Delete the Microsoft Authenticator method on your previous device and Click "OK".
- You will be asked to relink the account to Microsoft Authenticator next time you sign-in.
Hope my answer helps. Feel free to comment if you still have any questions.
Have a good day:)
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