We use viber, skype or foocall.

viber is free but the number that you are going to ring should have viber app as well. http://www.viber.com/faq

skype is free but if you going to ring from skype to someones phone/mobile number then youve got to pay for their rates per minute. http://www.skype.com/en/rates/

Foocall is the same, we use it to call my husbands auntie living in OZ but it isnt free there is a charge as well but way to cheap than the normal international calls, for more infos bout foocall go to this link. http://www.foocall.com/why-cheap-international-calls

Hope that helps.