Before even trying USB_modeswitch there are two things to try. First, try to eject the usb drive that the modem is showing as! (on my modem it comes up as an optical drive)
Second try booting up with the modem already plugged in. Some modems start in modem mode by default if the system is booted while the modem is plugged in.
My modem uses a Qualcom chipset and I have to use
sudo modprobe usbserial vendor=0x17af product=0x6002
to get it to be correctly recognised. I added the line,
usbserial vendor=0x17af product=0x6002
to etc/modules to do this automatically! (note that the vendor/product id may be different for your modem so please check lsusb to get that if you need to go this route)