Okay, I have done a little more research, and some limited testing, on this matter. I am testing with my IBM ThinkPad T43 (1871-W8M) running eCS 2.0 Silver. I am not sure which version the PCCard/PCMCIA driver I have. PCMCIA.SYS is dated 11/11/2004. The following are my observations:
1) The order of loading for USB, and PCCard/PCMCIA doesn't matter, UNLESS you are using CBENABLE, which seems to want to be loaded before USB. I have no way to test this, so I guess it should be USB after PCCard/PCMCIA (which includes CBENABLE).
2) I can attach, and operate an ancient PCMCIA Ositech 5 of diamonds (33.6K modem). Testing does not include going to the phone line, since I no longer have a phone cable, but the AT commands return correct information. That works whether I load USB first, or second. I also have ACPI 3.18 installed, since there is some question about that.
3) I recommend not bothering with a USB modem. There are only a couple of models that will work, and they are very old now (of course, all modems are now old). See:
http://www.os2warp.be/index2.php?name=modemshome for what is known about various modems (if anybody knows any different, please fill in the submission form).
4) An external modem, from the above list, should work fine, if you have a serial port to work with. Many newer systems don't have a serial port. I do have a Belkin USB to serial adapter model F5U109, but I have never tried it with a modem (I use it for my UPS).
5) Last, but not least, I will recommend that you seriously consider not bothering with a modem, when you have WiFi access. WiFi is far faster, much easier to use, more reliable, and much more available than phone line access anyway. The only reason to even think about using a modem, would be for sending, and receiving FAX to a real FAX machine.