Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Thank you for providing the details. Based on the information you shared, I understand that your old phone was damaged and you have therefore lost access to the Microsoft Authenticator app. You are the sole administrator for two separate work accounts, and the loss of your MFA method now prevents you from signing in to either account. You are receiving error code 500121, and your attempts to contact Microsoft phone support have ended without reaching a support agent or resolving the issue.
I'm just another user here on this public forum, not Microsoft's support, so I'm not able to reset or unlock anything on your account myself.
Because the phone line is not supportive, you can consider directly submitting a support request as a helpful workaround:
Consider signing up for a trial subscription to create a new tenant. Visit Microsoft 365 Business Plans and Pricing | Microsoft 365.
Once the new tenant is set up, you can access its admin center and submit a support ticket through Microsoft 365 admin center > Support > Help & Support to contact the Data Protection team regarding the previous tenant. Please remember to cancel the trial subscription after your issue is resolved, as this will help you avoid any accidental billing. You may prefer the following resource for detailed instructions: Cancel your Microsoft business subscription in the Microsoft 365 admin center | Microsoft Learn
I hope this helps point you in the right direction. If you have any updates or additional questions, feel free to reply back and I'll do my best to assist further.