Error Code 40351 — Phone number did not pass security checks during Microsoft 365 Developer Program registration

Question

Hi, I am trying to join the Microsoft 365 Developer Program and request an Instant Sandbox or Configurable Sandbox, but I am unable to pass the phone verification step.

During verification, I consistently receive the following message:

We can't continue with the signup process for the following reason:
Your phone number did not pass security checks.
Code: 40351
MS-CorrelationId: <value>

From what I found in the Microsoft Q&A and GitHub issues, Error 40351 seems to be related to backend verification filters that only Microsoft Support can fix.

Could you please assist in enabling my account to proceed with phone verification, or manually validate my enrollment into the Developer Program?

Thank you in advance for your help.

Answer 1

Hi @Mancini, Andrea 

Thank you for posting your question in the Microsoft Q&A forum. 

The error code 40351 usually indicates that the system was unable to verify your phone number during the security check. This can happen if your country or region is not on the supported list, or if there are issues with the phone number itself. 

To confirm whether your location is supported, please refer to Microsoft 365 International Availability.

If your country is listed but the issue persists, we recommend contacting Microsoft Developer Support for further assistance through Services Hub.  
Please note that Services Hub is intended for paid support tickets and access is subject to certain conditions. You can find more details here Services Hub sign in FAQs | Microsoft Learn. 

If you need to create a new developer account right away, you may want to try registering with a different phone number, such as a work mobile or secondary line, to complete the process successfully. 

Thank you for your understanding and cooperation.  

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Answer 2

Error 40351 indicates that the phone number failed Microsoft’s security checks during verification. This is similar in nature to cases where numbers are blocked or flagged with a “bad reputation” and cannot be used for SMS verification.

For Microsoft 365 Developer Program signup specifically:

1. Ensure the phone number meets the program requirements
   • Use a valid mobile (cell) number, not a VoIP number. VoIP numbers are not allowed for verification.
   • Only one Microsoft 365 Developer Program account can be associated with a given phone number.
   • If previously used for another Developer Program account, try a different valid mobile number.
   These requirements are enforced during the “Send code” step when setting up an instant or configurable sandbox.
2. Avoid repeated verification attempts
   • Excessive or repetitive requests for codes can cause Microsoft systems to temporarily block sending verification codes or to fail security checks.
   • Wait before retrying if there have been many attempts in a short period.
3. Check for conditions that cause blocking
   • Microsoft may temporarily block verification if there is unusual activity or heavy compromised traffic from a region. In such cases, the block is lifted automatically after some time.
4. If still blocked, contact Microsoft Support
   • Public forums and normal support channels cannot override security checks directly or send verification codes.
   • However, for issues where a number is blocked (similar to “bad reputation” cases), resolution has required internal action by Microsoft support/data teams.
   • Use the official support entry points:
     • For sign-in/verification issues, start with the sign-in helper: Sign-in helper.
     • For further assistance, go to Contact Microsoft Support, describe that phone verification for Microsoft 365 Developer Program fails with code 40351, and include the MS-CorrelationId shown in the error.
   Support agents cannot send codes or directly change account details, but they can escalate verification and MFA issues internally, as seen in similar cases where a blocked number or MFA configuration had to be reset by Microsoft teams.

If the current number continues to fail security checks even after waiting and ensuring it is a valid mobile number, the only practical options are:

• Use a different eligible mobile number, or
• Work with Microsoft Support so the underlying block on the number/account can be investigated and cleared by internal teams.

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References:

• Set up a Microsoft 365 developer sandbox subscription
• Welcome to the Microsoft 365 Developer Program
• Troubleshoot Microsoft verification code issues
• Common problems with two-step verification for a work or school account
• Azure has blocked myphone number for SMS verification due to a “bad reputation” flag - Microsoft Q&A