Your phone is most likely locked if you bought it from a carrier in the US. You used to be able to find people that could unlock them for about $25, thus allowing you to use a sim from anywhere in the world. I dont think the newer gen phones can be unlocked outside the factory as easily. Otherwise, the easiest way is to get a GSM-free phone. They should be about $50 or less, and then you can put SIM cards from whatever country you're in.