The call interface will show your FB image to the recipient. The call time is displayed on the upper left corner and you get the option to mute voice or to use loud speaker. You can also navigate to other features when the voice calling is on. You will of course require decent internet speed for this task and if you are attempting free calls while on the move, away from the comfort of your WiFi network, you will utilize your valuable data pack. To use voice calling, open a conversation thread on Facebook messenger and click on the menu button (three vertical dots) on the interface. Select the free call option from the drop down list and you are done.
With the introduction of VoIP feature on Facebook messenger too, the competition among chat messenger is now stiffer. Of course the best one for you is the one your friend(s) uses.