It is not possible from within the PBX itself. You will need to put an external device on your analog line called a FaxSwitch with Auto detect. The FaxSwitch answers the call on the first ring, and listens for a tone ("CNG") that identifies an incoming fax.
You should be able to pick one up at Bestbuy or maybe even radio shack. Just search Google for one.
It has modular RJ-11 recepticles on it, so if your phone isn't close to the fax, you will need to make some wiring modification in your switchroom. Its easy!