Facebook is testing free voice calling in the United States.
Mark Zuckerberg's social network has begun the limited test using its Messenger app for the iPhone.
Currently the tests are restricted only to Canada and the US, and only work on the iPhone.
In just the same way as Skype, Google Chat and other voice over IP services, Facebook calls use either WiFi or cellular data and not phone call minutes.
Phone companies are worried that users will start to bypass phone calls altogether, and think of their operator in terms of a so-called "dumb pipe" in which service is basically inter-changable.
Some sites speculated that the service is either a precursor to the long-rumoured 'Facebook Phone' - or the phone itself.
Business Insider said that the call quality was "amazing" and declared it a "great move" by the website.
Facebook has not yet commented on a potential UK launch.
The news comes days after Facebook announced Graph Search, which allows users to sort publicly available information on its site.
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