"I don't bring my personal phone when traveling, because last time my 700$ iPhone was broken by stupid CBP agents" - that, or any reasonable excuse should work.
EDIT:
"I bring dumbphones when traveling because battery lasts two weeks, unlike iPhone"
"I heard that there are lot of thieves in the country I visited, so I was afraid to bring my real phone"
"I heard that airport X-ray machine can brick iPhone"
"I really like cheap, Chinese phones, they are so cute"
"iPhones are for hipsters, I prefer cheap, practical devices"
possibilities for excuse are endless. And, as the old rule goes, the stupider the excuse the better - if you can convince CBP agents that you are harmless idiot you win.