Vatican City is the smallest sovereign state in the world, and you will arrive on foot — up Via della Conciliazione into St Peter's Square, or through the Musei Vaticani entrance on the north wall. There is no border post and no arrivals hall. One square kilometre holds the basilica, the dome climb, the Sistine Chapel and the Swiss Guard, and the queue for all of it starts before the doors open.
That geography shapes what you actually need from a phone here. Timed-entry tickets, audio guides and the museum map all live in apps you will want loaded before you join the line, and much of the visit is spent indoors inside Renaissance masonry, where a signal that was fine in the square becomes unreliable in the Raphael Rooms. Download what you need in advance, and treat the dome terrace as the one place you are guaranteed both a view and a bar of signal.
