Make sure the one you are calling has installed and registered JusTalk.
Go to the Friends tab, and tap someone you are going to call. Choose Voice Call or Video Call;
Go to the Calls tab, and tap the Keypad button on the upper right of the screen. Enter JusTalk number you are going to call and choose voice, video or Out Call button.
Tap the main red button in the middle, choose someone you are going to call. Choose Voice Call or Video Call.