I use modem C-motech 550

1) add in modules.lst (after all nics, befor iptables) usbcore usb-uhci #or usb-ohci acm

2) change serial port in config SERIAL_PORT="/dev/usb/acm/0" PORT_SPEED=460800

3) for my modem need in config INIT_MODEM="AT&F0E0V1S0=0"

4) configure login/password/telephone in config

Enjoy!

Anatol Ivanov, Russia