A newly released version of the iPhone app Facebook Messenger (free) lets you make voice calls for free. So if you're within a WiFi hotspot, you can make calls without using up your minutes. It also works via cellular data, but then you're using up your data allotment. iMore walks you through the steps of using this new feature, complete with screen shots. The confusing aspect of this feature is that you must first initiate a conversation via a regular text message. But once you've done that, you simply tap the information icon at top right. This brings up a window that shows the option for making a free call to the person. You simply tap the Free Call icon to make the call. In order to receive incoming calls via Facebook Messenger, you need to have the app running or need to have its Push Notifications enabled so that you know a call is coming in.
It only works in the U.S. and Canada, and there's no option for video calls.