In tennis, a Call is a decision about whether a ball landed in or out of bounds.
Players make calls on their side of the court during matches without officials. They should call "out" loudly and promptly if they see a ball land outside the lines.
In professional matches, line judges and chair umpires make the calls.
Players can challenge calls in some tournaments using electronic review systems. Good sportsmanship requires players to make honest calls, even against their interests. John McEnroe was famous for arguing calls, while Roger Federer is known for his fair play.