If I travel to a country that solely uses gsm network and I have an iphone 4s from Sprint, will I still be able to use the iphone? It says on the apple website that the iphone 4s is a world phone (the reason I am getting it). Will I be able to test it out here by putting in a sim card from said country and would it work despite not having signal? Or would I need to unlock it? Where would I unlock it?