As in the Messenger app, voice calling within Facebook for iOS can be accessed by tapping on a friend's name in the chat menu and then tapping on the info button, which reveals the option to make a free call.
Both parties must install the newest version of Facebook to activate the feature. Facebook's latest update also adds newly redesigned Like, Share, and Comment buttons.
Facebook is a universal app that can be downloaded from the App Store for free. [Direct Link]-New button design makes it easy to like, comment and share posts in News Feed.
-Improved buttons to like, comment and share posts
-Share button to re-post stories from your News Feed now available in all languages
-Call friends for free right from Facebook (US and Canada)
-Free calling uses your existing data plan.
Article Link: Facebook iOS Update Adds Free VoIP Calling in US and Canada