If it’s an Apple phone, it needs iOS 15.0 or later for the
DCL app. Look for an unlocked used phone.
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Some of these (like 6, 6 Plus, SE first generation) cannot go past iOS 15.0. I have no idea how often the app would change minimum system operating system requirements.
I see refreshed, unlocked iPhone SE 2nd gen phones on
Amazon for $100-125. Still not cheap, but if your kid will need it for more than a cruise in the next few years, this could be a good choice.
When I wanted a new iPhone a few years ago, my niece, who worked at the A store, recommended the SE 2nd gen. It is a solid phone, but lacks some of the features of the $1K+ models. The camera takes nice photos.
I just updated it to iOS 18 today. I do not expect to replace it for many years to come.
I’ll leave Android phone talk to someone who knows/ uses one.