I tried minicom but still can't get anything back from the modem. I tried both ttyS0 and ttyS3, nothing was sent back for a "AT" command.
In the old thread (where people were talking about RTM8000 on 2.4 kernel roms), they always referred to ttyS3. But from dmesg it seems the card is using ttyS0.
In /sys/devices/platform/akita-ioexp/sharp-scoop.0/pxa2xx-pcmcia/0.0/
there is a file called tty:ttyS3, which is actually a link to ../../../../../../class/tty/ttyS3
I can't find the relationship among these facts, But I suspect that something is wrong.