I'll bet its not a hardware phone. Rather a kind of Skype/Google Voice feature that will be added to its mobile clients. On your Facebook app for iphone example

I'll bet its not a hardware phone. Rather a kind of Skype/Google Voice feature that will be added to its mobile clients. On your Facebook app for iphone example, there will be a new icon called "Phone" in addition to the existing News Feed, Inbox, Events, Photos etc.