USB_ModeSwitch
Current 3G modems come with an integrated read-only pendrive (Option calls that "ZeroCD"-technology), that contains all drivers and software for installation. Unless these drivers are installed, modems are detected as pendrive only, because the modem-part requires activation by a so-called USB_ModeSwitch, which is performed by the drivers.
Unfortunatley very few devices carry Linux drivers in their flashmemory, so you will need to enable the 3G modem in your device by a script called USB_ModeSwitch, as explained here.