CBP can't deny entry to a US citizen. They could temporarily inconvenience you, or confiscate your belongings (or arrest you if you actually break the law)...

CBP can't deny entry to a US citizen. They could temporarily inconvenience you, or confiscate your belongings (or arrest you if you actually break the law)... but they would not legally be able to detain you indefinitely. A lawyer would have a field day with that. Since there's nothing actually illegal about refusing to unlock your phone, at worst you'd lose your phone while they attempt to crack it offline.