I'm pretty sure the US courts would view this as a breach of the Fourth Amendment of the Constitution of the United States of America. There needs to be "probable cause" to carry out any search. I don't see "You've got a phone" without any other evidence, as probable cause to search it. "The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."
Of course in the UK our Members of Parliament (MPs) will love it and rush through any changes to legislation needed to make it happen, whilst continuing to exclude themselves from any such searches.