I have same problems with my modem on 12.04. All you have to do are the following things:
First install USB_modeswitch on your computer. Try to download it from Software Center or Synaptic package manager. Also check for libusb in synaptic.
Now check whether you have /etc/mod_switch.conf or /etc/mod_switch.d file. Don't worry about their content, just check whether they are on your system or not. If /etc/mod_switch.conf is not present then you can create it manually.
Now check the file in /usr/share/usb_modswitch you will find a tar file in it which contain some information about your device. Look for your device file in the tar file, you will get it there.
To know which file you need, just run the command lsusb in your terminal to get the vendor id and product id respectively. In your file you will get the information like:
TargetVendor= 0x12d1
TargetProductList="140b,1506,150f"
MessageContent="55534243123456780000000000000011062000000100000000000000000000"
You will have you own TargetVendor, Targetid, TargetMessage content.
Now go to terminal and run gedit /etc/mod_switch.conf and write the following lines in it:
DefaultVendor= 0x19d2
DefaultProduct= 0x2000
TargetVendor= 0x19d2
TargetProduct= 0x0031
MessageContent="5553424312345678000000000000061e000000000000000000000000000000
Please make sure you put the right entry. Default vendor and product can be obtained from lsusb command. Target vendor, product and message content from the file I mentioned above. Save it and close the file.
Now open the terminal and run the following command:
sudo usb_modeswitch -c /etc/usb_modeswitch.conf