Oh, and for non-citizens: I entered the US with a partner that was canadian, we both refused to unlock our phones. She was denied entry, but they strip searched and physically groped both of our genitals before being released. Simply not wanting to enter the US was insufficient; the only way we could leave the building was by letting the cops handle our genitals.

They will use every option available to them to punish you for disobeying their commands to unlock, even if you are not legally obligated to do so.