I'm pretty sure my problem is in the PMFax Pro Settings. I'll probably have to search through Keller's various user guide PDFs to see if I can figure out what parameters are wrong.
I have both the Serial Cable and the USB cable attached. I also have
DEVICE=C:\OS2\BOOT\COM.SYS (1,3f8,4) (2,2f8,3)
DEVICE=C:\OS2\BOOT\USBCOM.SYS /M:3
in <config.sys>. The OS is ArcaOS 5.0.8.
The Ports displayed under PMFAx Pro's settings are: FMD1; COM1; COM2; and COM3.
The only non-Internet communications program I have on this computer is HyperHost, and it seems it recognizes the modem - though I don't know how to externally connect to test it.
As far as the Add program, I don't think this is an issue. Though I copied the PMFax Pro directory from an installation on another computer, IIRC it was just an upgrade from a license I had from Keller for an earlier version. It opens fine and allows updating of <fx.ini>. It's just that I can' send faxes or receive voice calls w/ it - something that was no problem when I was able to use an ISA card.
Like I said, I'll just have to drill down into the guide files, though I did just buy an unopened 3COM US ROBOTICS Model 5605 56K Voice Faxmodem Pro V.90 X 2 to see if this works better. (It's still an old model but newer than my Model No. 0525, and it seems Model 5605 is reported to work well under OS/2.)