traveling in india, you used to be able to easily purchase a prepaid sim card from airtel, and other providers i imagine. you could place the sim in your unlocked gsm phone and you had a local number. calling the u.s. cost about 7 INR/minute; calls to other airtel phones were free; local calls were 1 INR/minute or less; if you were roaming in another part of india, it was perhaps 2 INR/minute. today, 1 US Dollar is worth about 50 India Rupees.
at some point in the last few years, this is no longer an easy proposition. today, you have to be a local resident in order to purchase a prepaid sim so that there is a record of who has purchased a sim and where they can be located. driving this is an increased use of mobile phones by those engaging in unlawful practices.
even when i had an indian passport, i tried to purchase a prepaid sim in bangalore (in the state of karnataka) when i was there on business trip. the airtel store would not sell it to me because my passport listed an address in new delhi - it did not list a local address.
you can have someone residing locally purchase the sim card for you. they will need to bring their id and passport sized picture and fill out an application.
the other option is to roam with the sim from your home country, which isn't exactly cost effective if you're going to make more than one or two phone calls.
always interested in ways to stay in touch
Budget Travel featured an article here: http://tinyurl.com/ddank7
with some interesting tricks (including the google voice mentioned in OP's post)