Your phone needs to support "GSM" first, its a hardware thing so your specific phone either has it or it doesn't. I don't know much about plans but there probably is n international plan, you will have to call your specific mobile phone carrier to find out more info, it will depend on what carrier you use.
edit: sorry didnt see that you said u have an iphone. iphones do have GSM so it should work. You might still need a different plan