What is the proper way to address a lady...?

in a formal/business situation? for example you have received business correspondences with her 1st & last name and later you need to talk to her on the phone, not knowing her maritaql status, how would you address her? Using the 1st name would seem inappropriate in this type of setting but idk.

Ms. followed by her last name. I'm in this situation all the time.

Ms whatever her surname is. Then she might say, call me Mary or Mrs X or whatever.

Ms._____

Ms. is always appropriate. After that, she can tell you how she would prefer to be addressed.

Madame works too. (perhaps sounds a bit more elegant, too)

Always Ms. until you know she is Mrs. or Miss. In fact, I've been both and have always appreciated Ms. It's presumptuous to use the first name until she gives you permission (I've done this, too, asked people to use my first name).

It is appropriate when asking on the phone to use their complete name such as "May I please speak to Jane Doe". If you know their marital status then "May I please speak to Mrs./Ms. Doe.

If her name is Ann Brown you call her Ms Brown. You're correct in saying that calling her Ann is inappropriate. If she wants you to call her anything else then she'll tell you.

Disregard these people with their foolishness hap. There's no Ms. !! It's either miss ro mrs. ____. Other wise ma'am is appropriate.