Facebook members with Messenger for iPhone will find a "Free Call" button located on friends' contact info pages inside the application. Tap the button, and the app will dial the contact over wi-fi or your phone's data connection.
If anyone needs to understand why carriers seem less concerned about your minutes and more concerned about your data use, this is a prime example. Instead of using your minutes, it's using your data.
Thus, while the calling functionality makes Messenger into more than just a chat or SMS client (Trillian, WhatsApp, Kik, iMessage), Facebook is potentially a pain in the side of carrier income. Of course, it was already a pain, as all chat clients are: The cash cow for carriers is SMS.
When you call someone, a push notification appears on their screen that says "So-and-so is calling. Call quality is reportedly on par with VOIP competitors such as Viber, Vonage, and Skype. Facebook's key advantage: a huge user base to draw Messenger for Facebook users and callers from.
Speaking of Skype, Facebook has had a partnership with Skype for video calling inside the social networking giant's website since the summer of 2011, but Messenger still lacks video calling. Meanwhile, the company has not provided information about a possible upgrade to its Android app, an international rollout, or VOIP calling through Facebook.com.
We'd expect, at the very least, that an Android upgrade should be coming soon.
Messenger for iPhone with the free calling is available now to U.S. users and does not require an app update.