In order to make voice calls, you need a Facebook Messenger app (4.0+).
Learn-How to Make Free International Calls
Method 1. Using Facebook Messenger App
Simply follow the below steps carefully to make free international calls using FB app.
Step 3- Now locate the Call button on the top right corner. Make sure that it is blue. If it’s black, it means either the contact is not online or is running an old version of Facebook Messenger.
Method 2. Using Normal Facebook App
If you don’t have Facebook Messenger installed on your phone, then don’t worry, you can still make and receive voice calls using the Facebook app. How?
Step 2- Now select Free Call. That’s it.