Facebook didn't have any mobile feature to announce at the big event yesterday, certainly not a phone, but it is working on voice calls and now it's making them available to all iPhone users in the US via its Messenger app. Facebook was already testing VoIP calls with some users, so this isn't much of a surprise.
But it is a rather big move, one that could prove powerful if Facebook manages to double down on this. Messenger is quickly becoming a capable app with a focus on communication and keeping it separate from the main Facebook app makes it possible to retain that focus while also piling on features.
The voice feature only works inside the Messenger app and it's free. You do need a data connection to use it of course. While the iPhone app is the first to get it, you can expect Facebook to roll out voice calls, in the US at least, more widely soon enough.
Whether it will be able to take on the more established services, Skype and co., or not remains to be seen. On the one hand, one company can't do it all and do it well, on the other you've probably already got the biggest list of contacts on Facebook.
With voice calls already underway, you can expect video calls to be coming too, to compete with Google+ Hangouts.