"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.
It means he's talking to the phone; it wouldn't be used to express talking by means of a phone.
Why a fiú beszél telefononok" is not correct? I thought if the order is not SOV than the object marker (-k) has to be applied to the object.