Here's why:
* It lacks a verb: A complete sentence needs a subject (who or what is doing the action) and a verb (the action itself). "The boy on phone" only has a subject ("the boy").
* It's incomplete: We need to know *what* the boy is doing on the phone.
Here are some ways to make it a complete sentence:
* The boy is on the phone. (Adding a verb)
* The boy on the phone is talking to his friend. (Adding a verb and completing the thought)
* The boy, on the phone, smiled. (Adding a verb and showing the action)