Kernel bug, workaround in 3.1.10
PCMCIA Serial and Modem Issues
modem doesn't work until setserial irq 0 -- but works after (John Ruttenberg)
Date: 2000, Feb 04
From: David Hinds <firstname.lastname@example.org>
This turned out to be a kernel bug, but a workaround is included in
the 3.1.10 driver package.
to: "Kernel bug, workaround in 3.1.10"