The ports on the phone are only for attaching RJ-9 headset cables and Electronic Hook Switch (EHS) cables, or connecting an optional "sidecar" containing more buttons. This is a purely digital (well, except for the handset/headset) IP phone. There is no analog telephone line input or output. That's why the phone sounds so great -- no multiple transitions between digital and analog signals.
The modern way to deal with faxing is to get a fax mailbox from a service provider, and/or just scan and email the documents.