It was common to have a provider ask a client to call her from a phone booth on the street where she had a view of you from her window.  The point of this was to determine if you were really serious about coming to see her or just wasting her time.  This was before the internet, etc.

Today the two-call system serves as a security device so that she doesn't have to reveal her room number to you until you are at the hotel or the neighborhood, etc.

in the neighborhood, or name of hotel.  The second gives you either the exact address and directions, or the room number.

LLAP,
Swim

Without naming names or specifics...  gals have indeed checked the call point to be sure you are not followed...  that you are there.  
A gal I'd seen several times...  I arrived early (planned as I've been burned by traffic & sudden weather events).  After locating the callpoint, I withdrew to a coffee shop within sight of the location.  I figured 2 minutes to get to the door, but bade the call from my car outside the shop.  She answered & said "I don't see you!"  By then I was in the hotel entrance, which I said & she watched me park...  and opened the side door to let me in.  But it almost blew up in my face.  

I don't think it's discreet to sit outside for more than a couple of minutes.  So I prefer to do as I said.  With favs I tell them I'd at the coffee shop...  with new friends, I get to the location before I call.