Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello @james warren,
I understand that the Global Administrator is unable to login to the account due to changing MFA device and there is no alternative MFA such as Phone or Email.
If you can't contact Microsoft Support by phone, you can try creating a new trial tenant using the link below: Microsoft 365 Business Plans and Pricing | Microsoft 365 After you fill out and submit the sign-up form, you'll have access to the Microsoft 365 admin center.
Once you've gained access, contact the Microsoft support team by following the guidance in the following document to create a support ticket: Get support - Microsoft 365 admin | Microsoft Learn
When submitting a ticket, be sure to make it clear that the issue is relevant to different tenants and Data Protection Team will first verify your ownership of the account and revoke your MFA once confirmed. After that, you can set up the Microsoft Authenticator app again on your new device.
Please note that this is a user support forum. Only the data protection team can help you revoke a session and reset passwords, accounts, multi-factor authentication (MFA), and more.
After the issue is successfully resolved, remember to cancel your trial subscription by following the instructions in the following document: Cancel your Microsoft business subscription in the Microsoft 365 admin center | Microsoft Learn
If you have any further concern, please feel free to let me know.