For fax capabilities to work 100% you must have the exact driver written for your modem. So either winxp pro is using the wrong driver or your system is. Check these items first and if you still have troubles let me know and we can do some further diagnostics.
I have been using the WinXP (Pro) Fax Program for months with no problem. Suddenly, it just won't send a fax. I go through the whole "send" routine, and at the "finish", my Fax just disappears. Not in "outbox", not in "sent"; all gone.
I have uninstalled, re-booted and reinstalled, but to no avail. The fax "receive" works just fine and the modem works as well on ISP dial ups and phone dials. My "restore" points don't go back far enough.