If you are a MTNL Triband user and want to connect to the internet through USB, Ubuntu 7.10 will automatically detect the network through the USB.
However, one would have to add the DNS servers in order to get started. (Kubuntu 6.06 (and later), though, could do it out-of-the box - See the K effect ;) )
Please see the http://www.ubuntu-in.org/wiki/Broadband_Howto#MTNL_Triband page for details. (Somehow, pppoeconf never worked for me on Triband - not even on Kubuntu).