Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello,
Thank you for explaining the situation and the steps you have already taken.
Based on your description, this appears to be a Microsoft Entra tenant lockout involving an unavailable MFA method. Your situation broadly matches Scenario 3 in Microsoft's tenant lockout guidance:
Preventing tenant lockouts – Microsoft Learn
Regarding the B2B guest account, assigning it the Global Administrator role in a different tenant does not provide administrative access to the original tenant. Each tenant has its own administrative roles and permissions.
Since you already have an open support case, I recommend following up through the existing support channel rather than opening another case.
If the assigned support engineer cannot move the case forward, request to speak with their Team Lead or Team Manager (TL/TM). Explain that you have already attempted to contact support by phone but could not get through to an agent. Ask them to review your request and advise on the appropriate next steps for recovering access to the original tenant.
You can find Microsoft's official business support contact options here:
Contact Microsoft Support for Microsoft 365 business
Please note that this is a community forum where users help one another. Community members and moderators cannot access your tenant or escalate your existing support case on your behalf.
For your security, do not share your tenant ID, administrator details, or ownership documents publicly. Provide any requested verification information only through Microsoft's secure support channel.
Best regards,