recapped movie CONNECTED BY PHONE IN THE SAME HOUSE BUT 20 YEARS APART

Seo-yeon, 28, takes the train to the South Korean countryside to visit her hospitalized mom. Having lost her smartphone on the train, she finds a phone in her mom's house. She finds a hidden basement and Young-sook's 20 year old/Nov.1999 diary when she, too, was 28. Her shaman stepmom believes her stepdaughter will become a serial killer if let out and is very cruel to her. The two 28 y.o. women start talking together on the same phone in the same house but separated by 20 years. Can Seo-yeon's dad, who died then, be saved? Can history be changed? Should it?