I've been messing around a bit with the linmodems.org quasi-driver. Having limited success.
Seems to be some sort of problem with the version detection in the serial driver...but it insmods just fine if you "-f"orce it in.
If you're using a 2.4.17 kernel (I'm basing my starting point on Rasmus' image (http://php2.chek.com/~rasmus)), you might want to look at:
http://home.kscable.com/lbittle/ia1 for a pair of modules that *just might work*.
Short instructions: Get a 2.4.17 system up, get these modules where you can get to 'em, and do
insmod -f /path/to/module/lt_modem.o
insmod -f /path/to/module/lt_serial.o
Assuming /devfs, etc. (Rasmus' image!), you should then have what appears to be a modem on /dev/ttySLT0. Else, read the docs in the accompanying tarball.
Note: As of this time, I have not attempted to dial out...I've just 'catted' to-and-from the device, looking at the pretty "AT" commands. If someone actually gets around to testing this before I do, *please* post your results here!
As I mentioned, I've included the tarball that started me down the path...so go ahead and fix what I broke--and make this useful!