Student or Learner
I don't get it(
Q1 - While you were out, ____ Mr Davis called. He left his number for you to call back.
Why not without an article:
While you were out, Mr Davis called. He left his number for you to call back.
Or "a" means here that the secretary didn't know this Mr Davis?
It has the idea of 'a person who gave his name as (Mr) Davis'. You are right - the secretary did not know him.