This has been discussed in several other posts but I'm too lazy to point you at them...so, a couple quick points: you may want to get an unlocked GSM phone and use it in Europe/India. If you're on T-Mobile or Cingular, you can get your existing phone unlocked if it isn't, assuming it has the right bands (you'll need 900 and 1800, though you may be able to squeak by with just one of the two depending on where you are.) Otherwise, unlocked GSM phones are quite cheap on eBay.

Prepaid SIM card are ridiculously cheap in India. Airtel now charges something around 0.5-2c/min for calls in India and about 7.5c/min for calls to the US. Other providers are similar (I recommend Airtel because when I was there a few months ago, they were the only one that let you buy recharges when you were out of your home area.) At those rates, unless you're on the phone for hours a day it's so much easier to just use a prepaid SIM. Europe is a good bit more expensive; I'm headed to Europe for a couple months myself in a few weeks so I'm trying to figure out the cheapest option there... Callback might indeed be the way to go for high volume.