Straight from Mac Help. A great feature, BTW.
"Setting up your Mac to receive faxes
You can receive faxes on your computer and specify whether you want them saved to a specific folder, forwarded via email, or automatically printed.
To receive a fax, your computer must be turned on and connected to an open telephone line. You don't have to be logged in to receive a fax, but if your computer goes to sleep, it cannot receive faxes. For more information about preventing your computer from sleeping, click "Tell me more."
1. Open System Preferences and click Print & Fax.
2. Click Faxing.
3. Select the "Receive faxes on this computer" checkbox.
4. Type your fax number in the My Fax Number field. This is used to identify your computer when you send or receive a fax.
5. Choose the number of rings before your computer answers a fax.
6. If you want your incoming faxes automatically saved to a specific folder, select the "Save to" checkbox and choose a folder from the pop-up menu.
7. If you want your incoming faxes automatically forwarded via email, select the "Email to" checkbox and type an email address in the "Email to" field. You can also choose addresses directly from Address Book by clicking the Address Book button to the right of the "Email to" field.
8. If you want your incoming faxes automatically printed, select the "Print on printer" checkbox and choose a printer from the pop-up menu."
