When a speaker wishes to call attention to the long duration of the waiting, rather than to its completion, the speaker uses the present perfect continuous. Mary obviously wants to impress the other person with the length of her wait, so she opts for the continuous rather than the simple.
"” Aren't you through making your phone calls? You've been talking on the phone for two hours!
"” The union has been negotiating with the company for almost a year, but they're still far from agreement
You can always use the present perfect simple, but it doesn't have the same effect.