"I want to call my son."
Translation:Eu quero chamar meu filho.
You do not use preposition after "chamar". If you see that, it is a kind of special construction, but that is not mandatory.
What do you mean by "special construction"?
Sometimes you may see the preposition "a" after some verbs that do not require the preposition. This is usually used to emphasize the meaning or make it clearer.
so you could say both "chamar" and "chamar a" then?