I have never successfully received a complete fax with OS 10.3.x or 10.4.x.
Every attempt has resulted in receiving the 1st page with the fax being stopped after that first page.
All my attempts have been on an iMac G5 1.8 GHz (first version), and an iMac G5 2.0 GHz (second version), and a 1.25 GHz Mac Mini (first version).
I have tried with various versions of OS 10.3.x and 10.4.x as they became available.
So far, I have never received more than the first page of a fax with any of them.
This has always been using the fax receiving program built-in to OS X, and using the built-in modem in each machine.
My phone lines are all excellent quality lines. They are all hard-wired, and are on dedicated incoming lines. I always get excellent connection speeds using modems, and never have any trouble receiving faxes with my real fax machine.
So, I would approach this idea very cautiously, and experiment with it extensively before you invest money into deploying it.
I never have had trouble receiving faxes with PC's or a real fax machine. So, I am guessing that something in the OS X system is less tolerant of the incoming fax.
Anyway, experiment cheaply first. Don't invest much until you know it works for you.