When the Phone Rings - Ending Explained

The series' greatest twists center on the identity of the 'Real' Baek Sa-eon. It is revealed that the protagonist was an illegitimate son of the family patriarch who was brought in to replace the 'real' grandson (the kidnapper), who was deemed unstable and supposedly died in a fire. The kidnapper is actually the 'true' heir who survived and seeks vengeance. Furthermore, Hee-joo’s mutism is revealed to be selective and psychologically enforced by her mother, who forced her to stay silent as a child to gain sympathy from the powerful Baek family. The finale reveals that the family patriarch was responsible for the death of Hee-joo's brother, a fact that nearly breaks the couple before they choose to sever ties with their families entirely and start over with new identities.

Some critics interpret the series not as a romance, but as a political allegory for the silencing of the South Korean populace by elite families. Under this reading, Hee-joo's mutism represents the suppressed voice of the public, and Sa-eon's name change represents the necessary death of the 'old elite' for progress to occur. Alternatively, some audience members view the 'Argan' arc as a psychological manifestation of the characters' need for extreme environments to finally feel 'safe' enough to speak.