Hi @IBN ,
From the description and the error message you shared above, seems like the phone number you selected is not of the correct type that can be used to assign to a resource account.
According to this document, the phone number assigned to a resource account needs to be a "service number" rather than a "user number". Noticed from the first screenshot that your number is from Operator Connect, right? If this is the case, please contact your provider and see if they are able to convert it into a service number.
For more details about the telephone number types supported in Microsoft Teams, hopefully you can find the article below helpful:
Manage telephone numbers for your organization
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