A US citizen has an almost unlimited right to enter the USA. The border search rule permits officials to search for things that are illegal to import. A blank phone is not illegal to import.
A non-citizen who arrives with a blank phone might be denied entry because it seems suspicious.
well OK, luckily my checking of media informs me no citizens have been stopped and held in custody by immigration without due process or any particular grounds, especially recently, so it all sounds good.