Cheap Android phone. Temporary, cheap SIM (or none—you can get one where you're going) with a different number from your real one. Only way to go. Bonus that it's 100% reasonable to do this because you don't want to risk having your main phone stolen/lost, too, so it's not even a weird (suspicious) thing to do.
Eventually, this might be seen as non-suspicious. Maybe. I hope so. It's quite prudent.
BUT for now it's also something only a very, very small percentage of the population would consider doing and then actually do. Border agents would know this, empirically. So if you're doing something that's highly unusual for the general population then you are, unfortunately, doing something weird and suspicious.
TL;DR: it doesn't matter that it's reasonable; it's still fringe and, sadly therefore, suspicious
