Turn Off Data
If you don't plan on using data while abroad but plan to pack your phone, there are two steps you need to take before departure: Turn off cellular data and turn off data roaming. You'll find instructions for doing so on an iPhone here, and here for an Android phone. Contact your cellular provider for further details on shutting off data. Fail to shut down the automatic downloads that bring emails, program updates, meeting notifications, and other data to your phone and you'll likely see some very expensive roaming charges on your bill at the end of the month.