An owner or admin needs to go to the Users Page. 

Click your name on the top right corner and select "Users"

Click the Edit icon (pencil) next to the user you need to add or edit the phone number for.

Next, select the phone number to assign to them from the "Assigned Number" drop down box and select the number you want assigned to them. Users have to be assigned to a phone number. Admins have access to all phone numbers on the account but still need to be assigned a default / primary number to use.

Scroll all the way to the end and click "Save" once done editing the user.

If you are an admin or owner and need to change your own primary phone number, check out this article HERE.

More about the different types of users and roles explained HERE.

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