search
For the call to be free both you and contact you are calling need to have application and have stabile internet connection, then make free call through the application.
Note: If your contact has application installed icon 
will appear next to his name
Open Contacts tab 
Choose from the existing contacts the one you would like to call
Click on the phone 
icon in contact to initiate the call
OR
Open Contacts tab
Click on the search icon 
in top right corner
Type the name of contact you want to call
Click on the phone icon in contact to initiate the call
If the call is not over the application it will be switched back to regular call