What is the ruling regarding a wife who prevents her husband from accessing her phone and sets a password for it, claiming privacy?

Each spouse has their privacy. A wife may conceal her phone and set a password, and so may the husband. The default is for the husband to assume the best about his wife and have good faith. It is not permissible for him to have ill suspicions or spy on her unless there is a cause for doubt, as Allah Almighty says: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy." Spouses must build trust and mutual respect and avoid situations that give rise to suspicion.