the key to avoiding big unexpected charges (especially without an international calling plan) is to always have Date Roaming turned off....on an iPhone this setting is found under: Settings-Cellular-Cellular Data Options....this way you don't have to worry about remembering to completely turn off/on Cellular Data each time you travel

we've never had a problem with unexpected data charges during our yearly trips to caribbean and europe...for voice calls, we either use WiFi Calling, pay the roaming per minute rates, or buy a local SIM