"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.

Why does anyone feel it's necessary to explain why they have the phone they have? There are plenty of people who use cheap phones, and not just as decoys. Why would they exist otherwise?

Almost all of those are lies. Lying to a federal agent is a felony.

I really have a dumbphone because the battery lasts more than one week..

So it wouldn't be a problem, but if you carry a dumbphone, hand it over and then they search your luggage and find another phone, you'd be in deep trouble.