Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hi Steve Wiggins,
Good day.
Thank you for reaching out to Microsoft community. I am following up on this thread.
According to your description "My customer went to register their tenant name for a first time and it states that it is not available"
I would like to share some more specific information with you, for your given scenario, you may need to contact Office 365 support team via Phone support option, because they can check certain information from the backend side resources, and then they can provide possible information. Since we forum moderators do not have any authority to check any Business tenant information, and certain standard process is necessary to reach out to specific support team.
Here is official document: Find Microsoft 365 for business support phone numbers by country or region - Microsoft 365 admin | Microsoft Learn
Here is one another global support phone numbers: Global Customer Service phone numbers - Microsoft Support
You may need to contact our Microsoft Business support team on business days (Monday to Friday) and times: (select your country's region code).
When you call the hotline as phone support, you may need to request them to refer your case to the appropriate support team. You may need to press 1 or 1/2 on the voice message for support. The billing support team will provide accurate information.
I would really appreciate your precious time. Thank you again for your kind cooperation.
Sincerely
Darpan