These scammers aren't very creative, but they do know one thing: a large percentage of the world's computers run on Microsoft operating systems. See this wikipedia article for more info. They are basically just trying their chances because say 1/600 falls for it but with robocalls they can make that many calls in 10 min. So its just a #s game they are playing and unfortunately people fall for it. I have an iPhone and I always block these numbers. Lately I have started making up funny voices to answer unknown numbers and they always hang up. Make a game of it and "spam" them back.
Also for every "Indian" making these types of phone calls there are people of all backgrounds. This particular situation they have an operation and the underpinning technologies to carry out their fraud. The new trend is to somehow obtain an open phone # to spoof from your same area code, which tricks people into thinking they are getting a call from a friend or a business etc. As scammers continue to evolve, there are more and more tools to fight back. I like to look up the # online and usually will see what others are saying about the number.
If its on your mobile there are some other things you can do to protect/detect this. This Verge article is a good one. I'll probably be talking with my carrier and finding another way to handle the elevated amount of spam and robocalls I receive.
I hope that helps,
heirofnone