Unable to Verify Identity for Microsoft 365 Login – Error 399287

Hello @Louis Udeani ,

I’d like to clarify that I don't represent Microsoft, I do not have access to user sign-in logs, cannot reset MFA, revoke access, or contact Microsoft Support/Data Protection Team on your behalf. Please consider me an end user as well, providing guidance based on experience and documented processes.

The error code 399287 usually indicates that Microsoft has blocked SMS‑based authentication for the phone number or tenant due to a reputation or risk flag. This usually happens when Microsoft detects patterns such as repeated failed attempts or suspicious behavior. Once this block occurs, SMS verification fails even if the number is correct and previously worked.  

Firstly, try selecting "Use a different way to sign in" or "Sign in another way" on the sign-in page and use another available verification method, such as email or Authenticator verification.

If the issue persists, your tenant admin can resolve this by revoking the user’s MFA methods and requiring them to set up MFA again. Here’s how an admin can do that: 

• Go to Microsoft Entra Admin Center: https://entra.microsoft.com  
• Navigate to Users > All Users.  
• Select your user account.  
• Go to Authentication Methods.  
• Click "Require re-register multifactor authentication".  

This action clears out the user’s existing authentication methods, including phone numbers, Microsoft Authenticator registrations, and software OATH tokens, and prompts them to set everything up again during their next sign‑in. 

If the issue continues after the reset, the Global Administrator should open a support ticket so a Microsoft Technical Support Engineer can investigate further. Microsoft Support can check and clear backend blocks when needed. For instructions, you can refer to: Get support - Microsoft 365 admin | Microsoft Learn   

If you're an admin, please check to see if there are other admins in your organization who can help you. 

If you are the only administrator in this tenant, you will need to contact the Microsoft Data Protection team, as they are the only group authorized and equipped with the necessary tools and verification procedures to confirm your identity and restore administrator access. 

Please try to find the related hotline number to call the frontline and let them raise a ticket for you: Customer service phone numbers - Microsoft Support 

Depending on your country or region:

For some countries, when calling the support number, although there are about 30 seconds opening as something "you can visit the link...." But you can ignore this opening and wait until the option, then "1" as a business email user, "1" again for technical help.   

For some countries, it will be an automated conversation like:    

• First, when you call the hotline, it asks what kind of problem you are worried about.   
• Answered: authenticator.   
• A: What kind of product are you using?   
• B: Microsoft 365 for business.   
• A confirmation: education or company account?   
• B: For business
• A: Are you an administrator?   
• B: Yes.   
• A: Do you have the other admin in your organization?   
• B: No.   
• A: You need one.... Service request?   
• B: Yes   

Note:

• You may need to call multiple times before you are connected to a live support representative and can successfully create a support request with a ticket number for tracking. 
• If your subscription is from a partner or reseller, contact the reseller's support provider to help open a service request on behalf of you instead.   

If you encounter any difficulties in contacting the frontline support, please feel free to let me know. Thank you and I wish you a great day!