I've been using a Linksys Etherfast Lanmodem 56K (PCMLM56) and had a problem that was similar to the one posted, so I thought I'd post the solution I found in case it might be useful to anyone else.
Previously, I was running Red Hat 6.1 and the card worked fine right out of the box. But when I switched to Red Hat 6.2 the modem part of the card wouldn't work. I'm running Mandrake 7.1 now and found the same problem: the card is recognized properly, the serial_cs and pcnet_cs modules are loaded with no errors, and the modem gets assigned to /dev/ttyS2 with IRQ 3. This is all the same as it was before with Red Hat 6.1, but now with RH 6.2 or Mandrake 7.1 the ethernet works but the modem doesn't. No error messages are generated at all in /var/log/messages. Minicom can't talk to the modem, and if I do "query modem" from kppp I just get "Sorry, the modem doesn't respond." Also, setting the IRQ to 0 doesn't help at all. Other modem cards I tried (3Com and Xircom) still worked fine, so it appears that the problem was only with the Linksys.
Here's what I found to fix the problem (in Mandrake 7.1, using kernel 2.2.16, pcmcia version 3.1.14). In /etc/pcmcia/config.opts there's a line:
# Extra port range for IBM Token Ring include port 0xa00-0xaff
I found that when I commented out this line, the modem worked again.