You will need to add the modem's PnP ID to the PnP ID list in the serial driver. To enable Plug & Play support, compile a new kernel with controller pnp0 in the configuration file, then reboot the system. The kernel will print the PnP IDs of all the devices it finds. Copy the PnP ID from the modem to the table in /sys/i386/isa/sio.c, at about line 2777. Look for the string SUP1310 in the structure siopnp_ids to find the table. Build the kernel again, install, reboot, and your modem should be found.
You may have to manually configure the PnP devices using the pnp command in the boot-time configuration with a command like
pnp 1 0 enable os irq0 3 drq0 0 port0 0x2f8
to make the modem show.