You are right that many younger people (and not only younger people) primarly think of those little black rectangles as computers with several apps. As you say, in their minds, making voice calls in the classical way just happens to be one of those apps rather than the device's primary function.

But you are dead wrong about the word "phone". It still means those little black rectangles. If you primarily think of those little black rectangles as computers (or social media machines) then the word "phone" has shifted meaning to match that. If you punish a child by banning them from "using the phone" you will absolutely get the horrified reaction you'd expect.

Of course words vary in meaning throughout the world and maybe "phone" really does mean the classical phone app in your area, or in your daughter's social group. But that's exceptional, regardless of age.