So, YOU (as the secretary) should get the information, Ask the visitor the purpose of his visit, and get his contact details. THEN check with your boss AFTER the visitor left.

You see, in my position in our company, its crazy hectic, I would flip my lid as well if the phone keeps ringing every now and then by calls from sales agents with their sales pitches.

That's what company secretaries do: screen calls

I won't even disturb/call to MY BOSS' extension unless I deem its an urgent matter, it's an expected call, call from his MOM or Dad.

I hope you understand what I'm pointing out.