I bought a Hayes Accura years ago for $50 US, just in case. It even works on a Mac, with a usb to serial adapter, though at 33.6k, not 56k because the adapter was made for pda syncing.
Even getting isa modems to work in Linux can be a pain. I ran Red Hat 7.2 once and had to go through a box of ten isa modems just to find one that would detect and work. The usb USR serial modem that worked well with no special drivers in Windows would detect in RH but not work. I didn't have the skillz to edit the conf file to make it work and since it was a P2, it still had isa slots.