I see this mentioned a lot. As a visitor, I'm wondering what this means and what it's for.. you call them.. they call you back? or something like tha

You place your first call to get the general location (name of hotel or cross street).  Once you arrive, you call again for her room # or apt. #.
This is really a question for the Newbie Board.

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1 call, you make a call, setup appointment and get general area or hotel
2 call you get actual address or room

There are slight variations but pretty much the same

Then another example
1 call, you call make appointment and they give you all information needed to get there
2 call, to attorney because it was a sting and you got caught