He is mostly correct. One must master simple sentences before trying the more complex. But I think he is incorrect about accents. If one has a thick accent that prevents others from understanding his English, then one should work to correct that.
Also, he is missing the subtext in talking about "Where are you?" If I am supposed to be at a meeting and my boss calls and says "Where are you?" he is not really asking for my whereabouts. He means "you are supposed to be here."
So an answer "I'm on my way" is perfectly appropriate.