Are you going to activate roaming for calls? Let's try both
YES. What Sahikanjiyani said it's the way to go. As an extra precaution go to your mobile settings and make sure that data roaming is off.
NO. Then the best thing could be switching airplane mode on. Use your quick settings to switch WiFi on when need it (or GPS, Bluetooth, etc) and off when done. This way you'll save a lot of battery.