Buy new phone, put random things there to look real and give them that phone instead.
Or wipe your phone, restore backup from cloud on arrival.
Or wipe your phone, restore backup from cloud on arrival.
I'd wager the agents could legitimately (for some deranged, modern usage of that word) require that you either surrender your phone anyway or restore it immediately so they can review what you're clearly planning on bringing into the country.
It's easy to come up with new stasi operation techniques if there are no civil liberties.