Again, "he is calling the doctor" is a more natural and typical translation. Coincidentally, in this same lesson, the very next item / question is "Il contadino legge il giornale.", which I translated as 'The farmer is reading the newspaper.", and my answer was marked Correct. How is "legge il giornale" any different from "chiama il dottore"?!
"Il dottore" is a male doctor. Traditionally, a female doctor is "la dottoressa." There has been movement to call women doctors "dottore", but I would think they would still use "la" as in la dottore. It's common for words that can refer to either gender to use the correct article to indicate gender. For example, a software programmer is a softwarista. It can be both masculine and feminine so you would say il softwarista or la softwarista to indicate the gender of the programmer. A Milan (soccer) supporter is a milanista. Could be "il milanista" or "la milanista" depending on the gender of the fan.
I understand the question to mean 'why is it "he calls" and not "she calls"?'.
The answer to that question has nothing to do with whether it's a male doctor or a female doctor.
"Chiama" can mean any of "he", "she", or indeed, formal "you", and all of those possibilities are accepted. It just happens that the "he" sentence is the top of the list of accepted answers, and therefore shown first.
Sorry. I wasn't really looking at the sentance. It can be he or she, and if they haven't given any other information, both should be accepted. It would not be the formal You because it doesn't have Lei. It could be the imperative you, i.e. in English "Call the doctor" The second person imperative of chiamare is chiama.