An antagonist is a character who opposes the protagonist. They usually stand in the way of the protagonist's goals, either intentionally or unintentionally. Antagonists need not be villainous in nature; some are even sympathetic figures with understandable motives.
In Sorry, Wrong Number, there is more than one antagonist. In fact, every other character save for Mrs. Stevenson could be considered the antagonist, since everyone she tries to contact prevents her from achieving her goal: to stop the planned murder. For much of the radio play, the telephone operator serves this role, misunderstanding what Mrs. Stevenson wants of her when asked to redial the wrong number again.
The officials at the police department also serve an antagonistic role, assuming Mrs. Stevenson is a hysterical woman not to be taken seriously when she insists upon her hard-to-swallow story in a rude, angry manner. The hospital staff is also no help when she asks for a nurse, since the woman in charge of arranging such things is out to dinner when Mrs. Stevenson calls and not likely to return soon enough to be of any help.
Of course, the ultimate antagonist is Mrs. Stevenson's husband Elbert, who is revealed to have put a hit out against his wife. By the end, Mrs. Stevenson's goal has expanded: she wants to stop the murder to save her own life. Elbert is obviously in opposition to this, making him the chief antagonist in the story.