You can install any VoIP clients to make calls through WiFi, for example, Google voice, Skype, Vibrate (https://play.google.com/store/apps/details?id=com.viber.voip )..
The problem is that it is not free if you call a phone number. It will be free only if both of you are using the same service and are both online. Of course, it is far cheaper than mobile calls.
T-Mobile also offers WiFi calling for some plans.