Issue with SMS authentication

Hi Hoàng Nguyễn

Thank you so much for contacting Microsoft Q&A Forum.

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Based on your description, I understand that the issue you are facing which is being blocked from SMS-based login due to a "bad reputation", is related to Microsoft's Phone Reputation service, which is part of Azure Multi-Factor Authentication (MFA). Microsoft uses an internal service called Phone Reputation to assess the trustworthiness of phone numbers used for MFA. If a number is flagged as having a "bad reputation," it gets blocked from receiving SMS verification codes. Here are a few suggestions that may help resolve the issue:  

1.Use an alternative verification method: select “I can’t use my Microsoft Authenticator app right now.” This may allow you to verify your identity via email because SMS was failed, assuming those backup methods were previously set up.  

2.If the above method doesn’t work, you should reset your MFA. 

If you use a business account, do you know any other Global Administrators in your organization? 

• If yes, please ask them to follow this article to proceed to reset your 2FA so that you could re-register again: Manage authentication methods for Microsoft Entra multifactor authentication - Microsoft Entra ID | Microsoft Learn 
• If no, then you are the only Global Administrator and you are completely locked out from the tenant. So, only Microsoft Support team can help you in this MFA issue. You might need to contact them via phone service number: Customer service phone numbers - Microsoft Support. The agent will be able to create a ticket for you under the affected tenant and then transfer this ticket to Data Protection team who could help you to regain access. 

Please note that this team is always overwhelmed, so you might need to wait for them to call you, this should take 3-10 working days. I know that this is not a convenient solution to hear but it seems to be the only way, and it's recommended that you should have a secondary/backup Global Administrator to fix this issue on your own. 

I hope for your understanding and patience in this matter as I really want to help you. However, this issue is beyond of my control because I'm limited in my forum moderator role and cannot access the internal databases to help you directly. So, please kindly contact Microsoft Support team for the most efficient help. 

I hope you will be able to resolve this as soon as possible.

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