Sole Global Administrator locked out due to MFA error 399287

Dear @Elekidis Nikos,

First of all, I would like to clarify one point: this forum is intended for users to assist one another. As a forum Moderator, I have no authority or the tools to unlock MFA on your behalf.

Regarding to your issue, the error code "399287" typically indicates that the phone number used for SMS or voice call MFA has been temporarily blocked by Microsoft’s Phone Reputation system. This can occur after repeated verification failures or activity that is detected as unusual. As a security measure, when this block is in place, SMS and call‑based MFA verification cannot be completed, and the prompt will continue to fail even if the credentials are correct.

If you're the only admin, in this situation, the Microsoft Data Protection Team has tools and processes in place to verify identity and regain access to administrator accounts. Please try to find the related hotline number to call the frontline let them raise a ticket for you: Technical or Sales support phone numbers (for Work account, scroll down to find your region's number) 

In some countries, this is an automated conversation: First, when you call the hotline, they will ask you what kind of problem you are struggling with. 

Answer: Lost access to Sole Global Admin Account

A: What products do you use?

B: Office 365 for business. 

Verification: Education or company account? 

B: For companies 

A: Are you an administrator? 

B: Yes. 

A: Are there any other administrators in your organization? 

B: No. 

A: I need one.... Service request? 

B: Yes 

(Important Note: Sometimes, you would hear the computer-generated voice at first, please kindly wait for it to end, and if an agent is available, you would be directed to them and get your support. Depending on your country or region, 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.) 

As an alternative, you can create a new tenant account and submit your request from there.    

To set up a new tenant, please follow these steps: 

• Visit Compare Office 365 Enterprise Pricing and Plans | Microsoft 365, choose any plan and click try for free to start a free trial.    
• Follow the guided setup process to create a new account for a new tenant. 

Once your tenant is created, you should be able to access the support portal and submit your ticket referencing your locked account without further issues.

In the ticket's description, you could refer to this message

"Hello, I am currently locked out of my tenant because multi-factor authentication is enforced, and I am unable to complete the verification process. I am the only global administrator for this tenant, which means I cannot access my account or the Microsoft 365 Admin Center. To resolve this, I have created a new tenant and would like to be connected with the Data Protection Team to verify my identity and assist with regaining access. I am available to provide any required documentation or verification to support the recovery process."

Ticket Support: Log in the Microsoft 365 Admin Center using the newly created trial admin account (not the currently locked one) > Support > Help & Support. You can raise support ticket at: https://admin.microsoft.com/#/support/requests   

When you open a support ticket, please make sure to turn off the Support Assistant (AI) in the help panel.  

Several customers have reported issues when the Support Assistant is turned on  the system sometimes fails to display the “Contact Support” option.  

As shown in the screenshot, simply switch Support Assistant > Off, then type your issue description.  Once the AI is off, the “Contact Support” button will appear normally, allowing you to submit a ticket directly.

• After clicking on "Help & Support" button, you can type "I need agent support via phone or email". After that, you can click on "Contact support" 
• Choose contact method: "Phone" 
• Fill all the information in this contact support details, please describe your situation carefully so they will have enough information to help you. 
• Finally, click on "Contact me"
• After completing them, you will receive an email appointment for a meeting to review this issue professionally together. 
• For more details, you could refer to this article: Get support for Microsoft 365 for business 

Please remember to cancel the trial subscription once your issue is resolved to avoid any unintended charges.

Kindly let me know when there are updates or if you need further assistance. Any updates you’re able to share would be really helpful.

I'm here and happy to help! Thank you for your kindness and patience! 

If you have additional questions regarding this response, kindly click Comment.