Click on the fax icon in the main nav and choose "Numbers" from the sub nav. Using the dropdown make sure that you select "Owner". This will show all of the numbers owned by the master account.
Click the wrench icon to the far right of the number you want to assign. This will open a modal where you can assign a manager (sub account or master account) to the number. If there is already a manager assigned you can click the "Unassign Manager" button first. If no manager is assigned simply click the "Assign Manager" button...
and choose the appropriate account.
Important: Do not use the "Release Number" button to try and unassign a fax number. This will remove the number from your entire account.