Cannot sign in to Microsoft 365 work account — SMS verification error 399287

Good day Zaw, 

Just to gently clarify before going any further, this forum is separate from Microsoft Support. I don’t have the ability to access or investigate individual accounts or escalate the issue directly Microsoft Support because this is a user‑to‑user support forum. 

The error code 399287 generally indicates that Azure has blocked the phone number being used for SMS verification due to a "bad reputation" flag. 

Based on what you've described, the next steps will depend on whether there are other administrators in your organization who can access the tenant. 

If another administrator is available, I'd recommend contacting them and asking if they can reset the Multi-Factor Authentication (MFA) settings for the affected account through the Microsoft Entra admin center. This is usually the quickest way to regain access. 

For reference: Manage authentication methods for Microsoft Entra multifactor authentication - Microsoft Entra ID | … 

If you're the only administrator and you've been locked out of the account, you'll need to contact Microsoft's Data Protection team for assistance. They are the only team authorized to help recover access in situations involving the sole administrator account. 

You can find Microsoft's support numbers here: Contact Microsoft customer support | Microsoft Support 

Note: I understand that you have tried contacting Microsoft Support without success. However, please try the example scripts below to help ensure your request is directed to the appropriate team.  

When contacting Microsoft, be prepared to verify your subscription ownership by providing details such as your organization name, billing or payment information, registered phone number and alternate email address. 

Once your identity has been verified, the privacy team can reset your MFA settings. After that, you can set up your account again in the Microsoft Authenticator app or choose another authentication method to sign in. I also recommend adding additional verification methods, such as a phone number, an email or a backup authentication option, to help ensure you can access your account if your primary method becomes unavailable in the future. 

Tips for navigating the IVR:  

(When you call the support number, you may hear an introduction of about 30 seconds such as "you can visit the link...". You can ignore this introduction and wait until you are presented with the options. Then press "1" as a business email user, and again "1" for technical help.)    

IVR: What problem is bothering you? 

Client: Lost access to the only admin account. 

IVR: What product do you use? 

Customer: Office 365 for business. 

IVR: School or Business account? 

Client: Business account. 

IVR: Are you an administrator? 

Client: Yes. 

IVR: Is there another administrator in your organization? 

Client: No. I am the only admin in my tenant.     

IVR: Do you need to create a support ticket? 

Client: Yes. I need to create a ticket. Please send me direct to the Data Protection Teams. 

In case you are unable to contact the frontline support: 

Consider signing up for a trial subscription to create a new tenant. Visit Microsoft 365 Business Plans and Pricing | Microsoft 365. 

Once the new tenant is set up, you can access its admin center and submit a support ticket through Microsoft 365 admin center > Support > Help & Support to contact the Data Protection team regarding the previous tenant. Please remember to cancel the trial subscription after your issue is resolved, as this will help you avoid any accidental billing. You may prefer the following resource for detailed instructions: Cancel your Microsoft business subscription in the Microsoft 365 admin center | Microsoft Learn 

If your organization's subscription is from a partner or reseller: Contact the reseller's support provider to help open a service request on behalf of you instead. 

Additionally, here are some reference threads where I have helped other admins contact Microsoft Support, which may give you confidence in using these options: 

Global Admin locked out after iPhone replacement - Authenticator MFA cannot be re-registered 

Sole Global Administrator locked out 

I wish I could help more directly, but as a fellow user in this community, providing these steps is the best support I can offer. I hope this helps point you in the right direction. If you have any updates or additional questions, feel free to reply back and I'll do my best to assist further. 

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