I've lost my old phone, so I'm unable to access to the Authenticator App. This is the only global admin account for my tenant.

Because the only global admin account is protected by Microsoft Authenticator on a phone that is no longer available, self-service recovery from the portals is not possible. Recovery must be done by Microsoft support using the Data Protection / Tenant Recovery process.

Use one of these supported paths:

1. Contact Microsoft Data Protection support by phone
   • Call the Microsoft global customer service number for the region.
   • In the IVR, clearly indicate that the issue is with Authenticator and Office 365 / Microsoft 365 for business, that this is a company account, and that there is no other administrator.
   • Ask to create a service request and be routed to the Data Protection team to reset the admin’s MFA methods and restore access.
2. If phone support cannot be reached, create a temporary trial tenant and open a ticket
   • Sign up for a new Microsoft 365 trial tenant.
   • From the new tenant’s Microsoft 365 admin center, go to Support → Help & support and open a ticket.
   • In the ticket, state that the request is for tenant admin MFA reset / tenant recovery for a different existing tenant where the only global admin is locked out because Authenticator on the old phone is unavailable.
   • Provide the locked tenant’s admin UPN, tenant domain, and error details so the Data Protection team can verify ownership and reset MFA.

Once Data Protection verifies ownership, they will reset the MFA registration for the global admin so that new authentication methods (including a new Authenticator app on the replacement phone) can be configured.

References:

• Can't use Azure Multi-Factor Authentication to sign in to cloud services after you lose your phone or the phone number changes
• I have a Micrsoft business account and I am the admin and the only user registered under this account. I can't log in because i have replaced my phone and the authenticator doesn't work. - Microsoft Q&A
• Requesting a Tenant Admin MFA Reset - Microsoft Q&A
• Severity A Situation - Locked out of global admin account (MFA) - business down - Microsoft Q&A
• Global Admin Locked out - Microsoft Q&A
• I need to reset my mfa methods - Microsoft Q&A