sorry I don't have an answer for you, but, I suspect unless you provide more information, neither will anyone else. The two main things you need to provide are:
1. What exactly is this "bsnl 3g modem". Eg is it a usb device... perhaps a serial number or something (or maybe a link to a web page with more information).
2. What OS (including version number) are you running? Eg Ubuntu 11.10