Facebook has recently made possible for its Android or iPhone users to make free voice calls to your friends.
Facebook officially has not announced this fact to the users worldwide but has quietly made available to its Messenger app for Indians. The addition of a small "Free Call" button in the app is now available for Androids and iOS users.
This service was introduced in US and Cananda last year, now it has spread Facebook's video calling facility feature in India too.
So now when your Facebook Messenger window is open, you can call your friends for free.