OK, today I tried turning off Error Correction on my modem, and the fax appeared to have gone through.

In Windows XP, I did Control Panel > phone and modem > modem tab > [modem highlighted] > Properties button > Advanced tab > Change Default Preferences button > data protocol dropdown box select “disabled.” Normally the protocol selected in this dropdown box would be "standard EC," I think that must stand for "standard Error Correction." If Error Correction is set to "disabled," in the next dropdown box, "compression," the value is automatically changed from "enabled" to "disabled;" you can confirm this by closing the window and re-opening it.

As I mentioned in my previous message, I noticed before that there is a beep heard during the call - but with EC disabled, the beep stops playing after the modems "align," synchronize. There is another word but I still can't remember it.

Note that whether EC can be left on or needs to be turned off may depend on the the kind of fax device or modem on the other end, or how it is configured.