When you travel overseas, the last thing you want as a homecoming surprise is a staggering phone bill due to roaming charges.
Prevent wasting money simply by putting your iPhone in Airplane mode. Turning on Airplane mode disables all of the wireless phone features.
However, keep Wi-Fi set to On. This lets you use the in-flight Wi-Fi if it is available.
Caution: To be extra sure you don't incur data charges, go to General >> Network and set data roaming to Off.