Hi @Mohan1
here's what you'll need to do to use Wi-Fi Calling:
(1) Have an iPhone 5c, 5s, 6, 6 Plus, 6s or 6s Plus with iOS9 or later installed — remember to update your carrier settings when prompted to do so at the end of the OS installation. Your phone must be a Fido handset.
(2) Have your phone's Wi-Fi Calling feature switched on.
(3) Be signed up for a voice and data plan with a monthly bill. Prepaid service customers can't use Wi-Fi Calling.
(4) Have an LTE SIM installed in your device.
At this time WiFi calls is only available for postpaid account not not prepaid.
You can find out all the details for WiFi calling here.
As an alternative if you and the person in India have Whatsapp you can use that and call for free via WiFi.
