"A fiú telefonál."

Translation: The boy is speaking on the phone.

[GrowingViolet]

So this could be used to describe both the act of initiating a phone conversation (calling someone) as well as the act of holding the conversation itself (being on the phone)? In English I have only ever heard the equivalent verb "telephone" (or its shortened form "phone") describe the former (and then, at least in the US, only rarely; as "call" is tremendously more common). Seems like Hungarian is more versatile!

[vvsey]

Yes.
Another way "to call somebody" is "felhívni valakit". But it does need a direct object.

[TomkaDominique]

The boy is phonecalling ... ???

[jsiehler]

That would be understood in English, but it isn't normal. The given translation ("the boy is on the phone") is much better.

[ukulelefiat]

I have heard people saying they would telephone. Not always but common enough.