How I address physicians depends on the situation. If I am contacting a
physician on a professional basis with treatment concerns or if the client is
present I use "The dreaded ' doctor...'"
However, in informal situations many physicians have insisted that I call
them by their first name. The younger the physician the more likely they are
to insist that I use their first name.
I find the reverse is also true. If the doctor is talking to a client about
me or making a recommendation that they make an appointment for a consult
they call me Mrs. Davis. On a one-to- one basis they call me by my first
name.
There is a sense that  mutual respect should be acknowledged  in the
professional context.  I always ask my client's permission to use their first
name. Maybe I'm a bit old fashioned but I feel respect is greatly lacking in
a lot of other areas (not just medicine).