Most people prefer you call them by their first name. When was the last time someone instructed you to call them by their last name?

So what's the big deal?

Well, it's really not a show stopper, and there is minimal risk you'll offend someone if you do call them by their first name; however, when you call someone by their last name you are showing them respect. In essence you are saying to them, I respect you and you are important.