I am not sure why your cell phone bill has not changed much over the past few years. However, a quick Google search returned these results, and this information seems to be at least two years old:
http://in.rediff.com/money/2007/jan/10mobile.htm
http://money.outlookindia.com/article.aspx?sid=10&cid=13...
"Bharti Airtel is investing $2 billion in 2006-07 to ensure the 'death of distance', which, hopefully, will further bring down call charges by 15-20 per cent. Moreover, with sharing of infrastructure among operators and the reduced burden of revenue sharing for universal access service licence (UASL), basic and cellular mobile service providers will lower operating costs and the benefits may again be passed on to the consumer.
Bottomline: If you pay Re 1 a minute for a local call today, you may have to pay 80 paise for the same tomorrow. Talk is cheap, tech is chief."
Thats a 20% reduction in margin.