Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Please note that this forum is not Microsoft support, this is a user-to-user support forum.
As a first step, you may want to review Admin Takeover options if the domain is associated with a dormant Microsoft 365 tenant and you can still prove ownership of the domain through DNS records. This process can sometimes allow administrators to gain control of an unmanaged or abandoned tenant and then remove or manage the domain accordingly.
If the Admin Takeover process is not successful, the next step would be to work directly with Microsoft Support. You can contact them by phone or open a ticket via Admin Center.
I know you have tried phone support before, please try again with the script below. Please visit the following page and reach out to phone number via: Customer service phone numbers - Microsoft Support
From what I know, you will need to get past the initial AI system to reach the live support team. You can refer to the guide below for tips on how to get through more easily:
If you have already attempted phone support without success, and you mentioned that you have access to another Microsoft 365 tenant, you may consider opening a support ticket from that tenant's Microsoft 365 Admin Center.
When submitting the request, please clearly explain that you are raising the support case on behalf of the affected tenant that you no longer have access to, and that the issue involves a domain that is currently tied to an inaccessible or dormant tenant. If the support team is able to help you regain administrative access to the affected tenant, you should then be able to remove the domain from that tenant. Once the domain has been removed, you can proceed with adding and verifying the domain in your current Microsoft 365 tenant.
Important: Make sure the domain is removed from the tenant before deleting the tenant. A custom domain that remains attached to a tenant can continue to block its use in another Microsoft 365 organization until it has been properly removed.