I came across a newly launched service
www.actionvoip.com. For India this is currently giving one of the cheapest rates- landlines are 0.6p/min and mobiles are 1.1p/min.
However, please beware this is a betamax service who are very well known for increasing their rates very quickly. However, call quality has been generally good with their services. This service might be useful for heavy callers. But if you are infrequent caller- stay away.
There is option of calling from pc to phone (using a voip client) and phone to phone. For phone to phone they have a fixed connection charge of 5 cents per call but it is free for pc to phone.