What is the effect of the stage direction when Mr Birling puts down the phone at the end of the play

I think the final call gives Mr. Birling an actual wake up call that he is indeed culpable.

Only Sheila and Eric fail to agree that they share responsibility. As Birling mocks his children’s feelings of moral guilt, the phone rings.

He answers it and is shocked, revealing the play’s final twist: “That was the police. A girl has just died—on her way to the Infirmary—after swallowing some disinfectant. And a police inspector is on his way here—to ask some—questions—”