App for making free international calls?

Hey I am looking for an app that would allow me to make free phone calls by using wifi. I have tried many apps such as Line, Truphone, Kakao and many more, but all of these seems to have many things in common. Some of these apps only allows you to make calls if you EARN enough credits, and other you have to pay per minute.

So, is there an app out there that will allow me to make phone calls to landline or cellphone, in which I DON'T have to earn credits?