Skype is one of the most used app for making international calls, you can make free Skype to Skype unlimited calls, you can also make cheap international calls too.
You can download it via Windows Market Place and then you can make calls on 4G, 3G or Wi-Fi. You can Call, video call and instant message anyone else on Skype for free with Skype for your Windows Phone.
You can make call on portrait mode, invite or accept invitation, block contact and so on. But one big problem with it is that this app cant be run on background, means you can’t minimize and do other task with it.
Check out its official video :-
You can download it from HERE, its available in 18 languages.