I swear there was a movie 20 years ago of a time traveller taking a picture of Jesus that survived until today.

On a more serious note: I think this question is ill-posed. A smartphone being buried in certain layers in the earth might fossilize and endure millions and even a billion years. A phone in a region with rock or ice flow might be grinded down into fine sand within hundreds of years. A smartphone touching lava will be burned to crisp within minutes So it depends heavily on where that phone is dropped.

You also gotta ask: is it findable? Did it migrate deep into the earth where no one will ever dig? Did it land at the bottom of the ocean and buried by layers of sand, which in the next 6 billion years will never be completely removed? How would you find such a tiny item on a planet that it so large, if you were to pick a location at random, with 99% chance it would be uninhabited?