I have a Lava 720g 3G USB modem. The modem is supported by the usb_modeswitch distributed with Ubuntu 11.10 and 12.04 Beta.
But, despite that it doesn’t connect out of the box. Here’s a very simple way in which I got it working.
Open a terminal and type following commands in it
sudo modprobe usbserial vendor=0x1c9e product=0x9605
sudo usb_modeswitch -v 0x1c9e -p 0x9605 -V 0x1c9e -P 0x9605 -M “55534243123456788000000080000606f50402527000000000000000000000”