"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.

what you wrote means " the boy is speaking on the phone you speak "

It is, report it.

Jeszzus mik ezek a kérdések?? xD

You mean the plural marker -k?

that would be "a fiú telefonokon beszél".
"The boy speaks on phones."