Truphone's 'Call Me' application consists of a button that can be inserted in Facebook messages, posted onto someone's wall, or added to a Facebook group. In fact, it can be put anywhere on Facebook where attachments can be added. After that, your friends simply have to press the Call Me button to phone you free of charge.
Call Me
The Call Me application is available to all Facebook users, and you don't even have to have a Truphone account to use it.
For now this service is only limited to landline number witha possibility of mobile phones in future.
Via : http://www.tech.co.uk/computing/internet-and-broadband/news/how-to-use-facebook-for-free-calls?articleid=1508942064
