At first, the feature was only available to iOS users in Canada through the Messenger app, but now anyone on iOS in the US and Canada can call each other for free. Call can be made over WiFi or cellular connection, but it seems like it only works on the iPhone, not the iPad. It's expected that the feature will roll out to more countries eventually.
It's a cool feature, but it's kind of annoying that after pushing to make the Facebook Android app native, the company seems to be ignoring Android again. Voice calling still hasn't even come to the Messenger app on Android, but it has made the rounds on iOS and is in the main Facebook app. It's a shame that Facebook made it seem like the company was dedicated to Android only to go back to putting all of its resources into iOS first.
If you want to get the Facebook app for iOS, just jump over to the iTunes App Store.