This should be obvious but I can't see it. I have a prepaid Skinny SIM in an iPhone 5. Going to Australia for a week next month - and the best deal is a data-only roaming deal for $15. I can totally live without cellular phone and messages for a week - but how do I turn them off? I can easily do wifi only - but I want to have the option of doing wifi or cellular data - without the risk that someone will inadvertently call me or message me and cost me a fortune?