Click on the "numbers" option on the left side panel and click the settings wheel for the number you want to assign:
This will open up your number settings and allow you to change or assign a new manager (i.e. sub account):
Next, choose the appropriate account:
Important: Do not use the "Release Number" button to try and un-assign a fax number. This will remove the number from your entire account. Simply change the manager back to your master account.