How do they know the room number if they didn't call you from the car?

Never give that out until the last minute.

One provider I know even leaves the door open a crack so you can just walk right in without knocking, although I don't know if I like that idea.

I say, "Do not knock on my Door"

Every date, no matter where I am, I ask that they not knock on my door. I am always waiting at the door.
Please do not knock. That is my rule.
And they follow it.
Thank Goodness
Because I startle easily, too.
When I get the call that they have arrived, I give them my location and then I say Please do not knock.
I would never expect a knock on my door without a call first.

You simply say "do not knock" That's all.
You also need to make sure you are at the door, and answer it upon his arrival. If you are prepared, he won't have to knock.
In a prior thread you said you stated that you use the two call system. Be ready, and he won't have to knock.

God I cant stand that! Scares the hell out of me, makes my dogs start barking, and of most importantly, lets the neighbors know that yet another person is visiting my house. Finally had to tell people to call me once they get to the parking lot, and do not knock on the door. Has worked fine so far.

A Fav leaves the door open (I call when I am close).  I have instructions to walk in, lock the door behind me.  The other day a neighbor was in the yard...  saw me arrive.  I know she has a cover story so I knocked lightly, walked in (deftly locking the door while calling out) "Hello".  
But I agree, in general, the second call should be made upon arrival & should be met at the door to get the gent inside quickly.
skb