I have to install and configure the wvdail ppp dialer. Below are the steps explained:
Step 1. Install wvdial from the synaptic.
Step 2. Run wvdialconf, so that it can detect your modem.
Step 3. Type "sudo gedit /etc/wvdial.conf" in the terminal. Gedit will opened with specified file.
Step 4. Delete the semicolons before the "Phone", "Password", and "Username". The semicolons are there to make them as comments.
Step 5. Enter your Phone( #777), Username and Password after the equal sign, overwriting what is already there.
Step 6. Save the file.
Step 7. Run "sudo wvdial" in the terminal.
After that I am able to connect to the Internet by typing "sudo wvdial" in the terminal.