The first one sounds natural, it puts the focus on the phone. The boy sees a PHONE, not a CAR. I'd say both sentences are correct, but the second one sounds very weird. Even though the grammar is okay, nobody would ever say such a sentence, and I cannot pinpoint exactly why. In its current structure it means "A car is being seen by the girl, not the boy." I'd rather say "A lány lát egy autót, nem a fiú." = "The GIRL sees a car, not the BOY."
I guess the weirdness comes from the indefinite nature of the object. Not a specific car, just "a car". So why is it so important that we put it in the front? Does not make much sense. But yes, it is a correct sentence.
Let's change the sentence to a definite one:
"Az autót a lány látja, nem a fiú." - I think it sounds much more normal. It is emphasizing the definite, specific car. It is a good sentence. We can also say "A lány látja az autót, nem a fiú", of course.
Let's pick a similar sentence:
"Ki kér egy almát?" - Who wants an apple?
"Egy almát ki kér?" - it sounds weird
"A lány kér egy almát, nem a fiú." - The girl wants an apple, not the boy.
"Egy almát a lány kér, nem a fiú." - sounds weird
Change to definite:
"Ki kéri az almát?" - Who wants the apple?
"Az almát ki kéri?" - good
"A lány kéri az almát, nem a fiú." - The girl wants the apple, not the boy.
"Az almát a lány kéri, nem a fiú." - also good!