USB_ModeSwitch is a tool for controlling "flip flop" (multiple devices) USB gear like UMTS sticks.
Run dmesg in monitor mode:
Plug in the device and check if it gets detected as a new CD-ROM drive. If so, then you have a "flip flop" device and you need USB_ModeSwitch.
emerge --ask usb_modeswitch
USB_ModeSwitch is designed to work out-of-the-box. Plug in the device and let udev do the work.
If USB_ModeSwitch doesn't work out-of-the-box, search the internet for a matching config. Use lsusb to detect the device's vendor and product IDs. If you find a config, create a new configuration file using the device's vendor and product IDs, e.g.:
TargetVendor=0x12d1 TargetProductList="1001,1406,140b,140c,1412,141b,1432,1433,1436,14ac,1506,1511" MessageContent="55534243123456780000000000000011062000000100000000000000000000"
If you don't use udev, you have to run usb_modeswitch manually:
usb_modeswitch -c /etc/usb_modeswitch.conf
After switching lsusb should detect a different product ID.
If the right drivers are loaded, the proper device files appear. E.g. in the case of the option driver the /dev/ttyUSB* files.