Assuming you have a dialup modem already, you will also need a serial port and Gnome-PPP software
Most motherboards these days do not have a serial port. They do however have an internal serial header or connector (refer the layout diagram in the motherboard manual).
You can buy purchase a 16-pin DB9 serial port back-panel bracket for the 10-pin serial port header on the motherboard. http://www.scan.co.uk/products/40cm-startech-plate9m16lp-1-port-db9-serial-port-bracket-to-10-pin-header-low-profile
After that connect your modem to the serial port on the bracket. Then power the modem using an 9v AC-AC power adaptor. Then install gnome-ppp.
sudo apt-get install gnome-ppp
Use 172222 for the phone number. Uncheck wait for dial tone. Use firstname.lastname@example.org for the user name and netone for the password. There is no need to register for dialup access or create an account for it. The service is automatically available for all BSNL landlines.
Click on the Log button to check the progress. Unless you see that DNS servers have been assigned to your connection, you will not be able to browse the Net.