For a smartphone to be fundamentally unexplainable by relevant experts one would have to go back more than 100-200 years, I think. The first trans Atlantic radio transmission was in 1902, for example. Though I guess even the experts of the day might struggle to believe that the device is practically realizable, since it probably would defy many a conception of what is doable (if not theorically possible). There are some things now that seem impossible, which in 200 years might be commonplace. Idk, maybe residential grade or even portable nuclear power.