The Supreme Court has ruled at least a couple times the security concerns trump the 4th Amendment at border crossings. I think the last time it was a unanimous court that said "justified by the Government's paramount interest in protecting the border"

I suppose they could argue that smart phones (that didn't really exist when that case was decided) hold so much (and so much more personal) information that the issue should be reconsidered.

which was a unanimous decision requiring a warrant for a cell phone search after an arrest causing
the courts to place more restrictions on boarder cell phone searches than previous court decisions
on border security implied.

https://en.wikipedia.org/wiki/Riley_v._California

For international travel. I found it amusing because the phone I have is considered a burner, and I like it.
My best friend traveled to Russia a year ago and had some minor problem in Russia cause the border guys had trouble believing an American would not have a smartphone with her. She left it home because it was a company phone with all kinds of corporate goo on it and she didn't want the risk of losing it in Russia.