"The boy speaks on the phone."
Translation:A fiú telefonál.
"Telefonál" is a verb in itself, meaning "to speak on the phone".
"Beszélsz" is conjugated in second person singular, so it doesn't match the subject here.
You could say "a fiú a telefonnál beszél" -- "the boy is talking next to the phone", but that doesn't mean he's using the phone: he just happens to be standing next to it, while simultaneously talking to somebody.