You need to sign up for a QBOA US account to manage your US clients. Ask your client to help create a new one with the phone number and you can change the phone number to your own later.
Adding clients from a different region to your QuickBooks Online Accountant (QBOA) UK account isn't possible, Bdvorani9. I know an alternative option to help you out.
If you are an accountant, adding clients is exclusive to the same region. However, as a workaround, we can let your client invite you as an accountant from their QBO account as a primary admin user.
In addition, you might want to check this article to learn more about user roles and access permissions in QBO: User roles and access rights in QuickBooks Online.
If you've got other follow-up questions about inviting users to QBO, let me know by adding a comment below. I'm more than happy to help.
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