This was at ORD, which I've found (perhaps it's just one of the benefits of being in the midwest) to be a very friendly port of arrival for international flights. I was asked aside for an "interview" upon landing (I'm a US citizen living in Chicago) by two agents, who asked if I had a phone and then asked me to give it to them. I politely but firmly said "I do, but I'm not comfortable unlocking it." They asked why, and a I responded that it's highly personal and that I would be happy to help them in any other way, but I believed I would be within my rights to refuse them access. They didn't push it further, but I was detained for three hours or so while they asked pointless questions they already had the answers to. My phone didn't leave my person during that time, though at one point they asked for some contact information that I said was only available on my phone, which I unlocked in front of them, retrieved, then pointedly relocked and put away.