Voice over internet protocol or VOIP is a net based is a technology that allows you to make long distance or international voice calls using a broadband Internet connection.
Now if u want to save on calling costs u have 2 options
1. NET to Net calls or IP to IP calls where both ends are computers. Generally Free or add revenue based.
2. Net to Phone calls or IP to PSTN phone calls, where a person uses a computer/ ip phone to call a regular phone. Such calls are mostly on a pay based model where people are charged depending upon the country.
Easiest places of access for Voip in India are the sify iway cafes and other cybercafes which allow access.
Other way to do it is to buy voip cards from a host of providers like Mtnl , Sify (good) and Anyuser India(decent) Tata etc. This is preferable if u have a decent computer with a mic and hdphone and a sound chip supporting noise cancellation and broadband connection at home, but quality differs from provider to provider.
There are other methods like SIP based Vonage, but i dont know its legality under the Indian Telegraph Act of 1885 or the TRAI.