@Jithin
A bit of googling reveals that setting up BSNL broadband through USB in Ubuntu is a pain. Anyway, let’s see how far we can go.

Did you get a message which had something like “access concentrator of your service provider failed to respond”?
Probably you just need to update your drivers for the Modem to be recognized. I manged to get a set of drivers for your modem. You can access them HERE.
This Rar’d file has detail how-to on installing a package in linux.So that shouldn’t be a problem. I suggest you do a bit of tweaking and get back to me with a log of the outputs. That’s the only way we can move forward.

P.S : My domain would be down for maintenance on March14th form 05:00hrs to 17:00hrs