a Mr Davis

[GoodDay]

I don't get it(

Q1 - While you were out, ____ Mr Davis called. He left his number for you to call back.
a
the

WHY "a"?

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?

Thank you.

[5jj]

It has the idea of 'a person who gave his name as (Mr) Davis'. You are right - the secretary did not know him.