MFA & Verification Issues

Fix Phone Number Failed Microsoft E5 Security Validation (Error 40351)

Kushani Nimanthika Oct 10, 2026 868 views

Question details

Users encounter error 40351 indicating their phone number failed security checks during Microsoft 365 E5 Developer subscription setup.

Fix Phone Number Failed Microsoft E5 Security Validation (Error 40351)

Product: Microsoft 365 E5 Developer Program
Device & OS: not provided
Scenario: Setting up a new Microsoft 365 E5 Developer subscription and attempting to pass SMS phone verification.
Observed behavior: The system throws error 40351, rejecting the phone number used for security validation even if the user claims the number has not been used before.

Before you start

Ensure you have access to a legitimate mobile carrier network, as virtual numbers and VoIP services are generally blocked by Microsoft's automated security filters.

Solution 1Recommended

Use a Standard Mobile Carrier Number

Microsoft's automated security systems frequently block VoIP numbers like Google Voice to prevent abuse.

Microsoft enforces strict fraud prevention policies on Developer E5 accounts. Using a physical SIM card from a recognized mobile carrier is often required, as internet-based phone numbers fail the underlying risk checks.

Avoid VoIP services

Do not use Google Voice, Skype, TextNow, or other internet-based phone numbers for the verification prompt.

Use a fresh number

Ensure the phone number has never been used to verify a Microsoft 365 Developer E5 account previously, as Microsoft limits accounts to one per phone number.

Input the number format correctly

Select the correct country code from the dropdown menu and omit any leading zeros for the local phone number.

VoIP Block: Even if your VoIP number is completely new, Microsoft's system identifies the carrier type and will preemptively block the verification attempt.

Solution 2

Switch to an Alternative Network or Browser

Verification can sometimes be blocked due to network IP reputation or conflicting browser cache data.

Solution 3

Contact Microsoft Support for Manual Review

If all valid physical numbers fail, you may need manual intervention from Microsoft's support team.

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Frequently Asked Questions

Why does Microsoft reject my Google Voice number for E5 verification?

Microsoft actively blocks most VoIP (Voice over IP) services like Google Voice to prevent abuse and the automated mass-creation of free E5 developer accounts.

Can I use the same phone number for multiple E5 developer accounts?

No. Microsoft strictly limits one phone number to one E5 developer subscription account to enforce their single-account-per-developer policy.

What does Error 40351 mean in Microsoft E5 setup?

Error 40351 indicates that the phone number entered failed Microsoft's automated security risk checks. This usually happens if the number is a VoIP line, has been previously used for another developer account, or was flagged by their fraud prevention algorithms.

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