All USB modems are soft modems just like most PCI modems. But they all have some version of the AT command set. Whether you can find an AT command reference for a given modem is problematic. I asked tech support of the mfr of my current external serial modem (Accent division of D-Link) where I could find the ATCR for it, and the guy on other end of the line sounded like a deer in the headlights looks... Duuuuh just doesn't cover it.
. Most of them use the basic set, it's just when you get into the new extensions (like for the v.92 protocol), etc that there may be some unique commands. If you dig around on the driver CD that comes with the mode you'll usually find at least a partial reference.
. IAC, I'd suggest getting a real external serial modem. If your mobo doesn't have an RS-232 serial port, then get a PCI serial card too, set up the port, connect up the modem and you can do whatever you want. AFAIK, only US Robotics still sells a true hardware PCI modem now.
Anyhoo, go to
http://www.TechIMO.com/reviews/ and check out my review of the Accent modem. I found one on eBay for a guy and he never sent me any nasty message afterwards. So I guess he didn't think it was a POS either. They sell it at Office Depot - house brand (Aptiva or something like that... It's the best modem I've ever owned and if you read my review you'll know a lot more about it than the folks that are selling it.
.bh.