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.
Sign in to your QBO account as a primary admin.
Go to the Gear ⚙ icon and choose Manage users.
Select the Accountants tab.
If you invite an accountant for the first time, enter the accountant's email address, then select Invite. Follow the prompt to verify your account.
Select Invite if you're adding a second accountant.
Enter their name and email address, then choose Save.