In the gripping second episode of "When the Phone Rings," titled "The Birthmark," the narrative continues to unravel the intricate web of secrets and lies surrounding the marriage of Baek Sa-eon and Hong Hee-joo. The episode delves deeper into their tumultuous relationship, as Sa-eon's suspicions about Hee-joo's recent kidnapping attempt begin to mount.
The episode picks up with Sa-eon noticing the bruises on Hee-joo's neck and forehead, remnants of her harrowing experience. His confrontation with her in private is met with silence, as Hee-joo recalls his cold dismissal of the kidnapper's threat. Her resentment towards him grows, further straining their already distant relationship.
Sa-eon's curiosity about the kidnapping incident deepens when he asks for Hee-joo's car keys, intending to review the dashcam footage. Hee-joo panics, fearing that the footage will reveal her long-hidden secret: her voice has returned after years of muteness. However, to her relief, the footage is mysteriously erased, leaving Sa-eon's curiosity unsatisfied.
During a dinner conversation with his mother, Sa-eon inquires about the wedding day substitution of Hee-joo for In-a, questioning if anyone outside their families is aware of this deeply buried secret. His mother's assurance that no one knows raises suspicions about the mysterious caller's knowledge of their private affairs.
Hee-joo's life becomes increasingly restrictive as Sa-eon assigns her bodyguards who report her every move, further disrupting her independence. At work, she encounters Sang-woo, an old college friend, who expresses sadness over her sudden disappearance three years ago. Their brief but warm conversation brings back memories, and Hee-joo apologizes for her silence, a silence that Sang-woo understands due to his fluency in sign language.
Unbeknownst to Hee-joo, her bodyguards capture photos of the meeting and report back to Sa-eon. Seeing Hee-joo smiling with Sang-woo, something she never does around him, Sa-eon feels a flicker of jealousy. At home, Hee-joo faces pressures from her mother, who insists she wears provocative loungewear to seduce Sa-eon. It is revealed that Hee-joo married Sa-eon under duress, with her father's care in a prestigious nursing home promised as part of the marriage arrangement.
Hee-joo's bitterness towards her situation grows, and she continues to seek small ways to rebel. She escalates her own ruse as the "kidnapper" by calling Sa-eon and sending him a photo of her thigh, showing a distinctive red mole. Sa-eon, who has never seen such an intimate part of her body, is startled that the "kidnapper" knows this detail. Believing he's dealing with a perverted criminal, he engages the caller in a provocative conversation to bait them, helping his team trace the call's location.
A flashback to three years earlier sheds light on Sa-eon and Hee-joo's strained relationship. Shortly after their wedding, Hee-joo had tried to bridge the emotional gap between them, but Sa-eon dismissed her, reiterating that their marriage was a sham and warning her to stay out of his business. In the present, Sa-eon seems to soften slightly, offering her a position at the presidential office and even preparing her breakfast, likely as a way to ease his guilt over her recent ordeal.
Sang-woo, now reconnected with Hee-joo, becomes a recurring presence. He is scheduled to appear on the same news program where Hee-joo works as an interpreter. During an encounter with the host, Yoo-ri, Sang-woo displays his charm by protecting her from a swarm of fangirls, only for Yoo-ri to defend the fans' passion. Sang-woo's lingering feelings for Hee-joo become evident, suggesting that his reentry into her life could complicate matters further.
Amid these developments, Hee-joo visits her father at the nursing home, breaking down in tears. She expresses her desire to shed her current identity as "Hong Hee-joo" and return to being "Na Hee-joo," hinting at a past life she was forced to abandon. The strained dynamics of her family remain a mystery, particularly why her father's wife appears indifferent to his well-being. Sa-eon's professional life also takes a turn as he confides in a reporter friend about the threatening calls. The reporter speculates that such intimate knowledge of his family must stem from an affair. This theory unsettles Sa-eon, especially as the caller continues to taunt him.
During one such call, Sa-eon sarcastically tells the "kidnapper" to wait for him to act, sensing their desperation. The episode crescendos with a shocking sequence: while Hee-joo drives near Sa-eon's workplace, she hears a loud explosion. In a panic, she tries to reach Sa-eon using the kidnapper's phone but fails. She spends a restless night, only to wake and find Sa-eon absent. When he eventually returns, the tension reaches its peak. Sa-eon abruptly checks her thigh for the mole, suggesting he's begun to believe she might be involved in an affair. Hee-joo stops him, leaving their interaction unresolved.
The episode concludes with a flashback to the day before their wedding, revealing that Sa-eon switched brides to marry Hee-joo instead of In-a. In-a had threatened to expose family secrets unless Sa-eon canceled the wedding, but Sa-eon learned Hee-joo's mother orchestrated her daughter's marriage to a man nobody wanted. Sa-eon's decision to marry Hee-joo hints at a protective instinct, though his motivations remain unclear.
Despite his aloof demeanor, his actions suggest lingering feelings for Hee-joo, raising the question of why he continues to keep her at a distance. The episode ends on a cliffhanger, leaving viewers eager to uncover the truth behind the birthmark and the secrets that bind Sa-eon and Hee-joo together in a tangled web of deception and hidden desires.