A little over a month ago, Facebook added Voice over IP (VoIP) calling capabilities to its iOS app for users in the US and Canada. Now users in the UK have been added to the gradual rollout of Facebook's VoIP domination of the universe.
The VoIP capability works through Facebook Messenger, both in the Facebook and Facebook Messenger apps. After selecting a chat recipient in the app, tapping the Contact Info button (the "i" in the upper right corner of the app) displays information about the recipient including a Free Call button if they're able to accept a call. Both parties need to be online within the app for the Free Call button to be active.
The calls don't use voice minutes or rack up expensive long-distance charges; instead, it chews up your data plan megabytes instead if you're on a 3G/4G network.