I still use the ES56-PI modem driver @ http://tx.technion.ac.il/~raindel.
It has a 2.6 port up to 2.6.24, but it's not more current than that due to
serial/tty interface changes. And there's a challenge that it uses a binary
blob. :-( Best case would be to see if the vendor could open up like
Atheros (Thanks! Yea!) did. If not, I wonder if the blob could be minimalized
to the bare modem functions and the serial interfaces be brought out into open code.