Open up your web browser and go to https://service-officeathand.att.com and log in. (Use the Forgot password" process if you don't have the password. The login ID should be your SCI Email address. )
On the main screen, click "Settings" on the top row, then click "Outbound Calls/Faxes" on the left.
Select "Fax Settings" from the dropdown in the main window.
Once it drops down, fill in the contact info as desired. Change the Phone number to your direct number (You might have to search through a list of possible numbers). Do the same for the fax number and click save at the bottom.
Log out of O@H and close the browser. Your fax number should now show the correct return phone number when faxes are sent.