I'm not sure how to fully comprehend the ending timeline here. There are more questions than answers of course to the entirety of the call to begin with. But I guess the main two are:
1. How does the phone call connect from 1999 to present day? The viewers are suppose to take it as is I guess, no actual scientific nor paranormal explanation here, just happens as is.
2. So the timeline of the two characters continue to progress as is, there's no indication of the present Young-sook alerting herself in the past until the very end and that in itself is a whole conundrum because we see the mom throwing Young-sook over the balcony and then surviving so that Seo-yeon is able to find her mom alive at the cemetery, but then as they both leave the cemetery her mom flickers and disappears because Young-sook survives the fall also. Now if the mom survives with Seo-yeon, wouldn't they call the police or at least find out at some point that Young-sook didn't die so that she would remain a threat? We only have the past calling the present until the very end, where the one instance is Young-sook is calling herself in the past which is a reversal of this mysterious phone communication. How does this happen? Lets say she figured it out and can call herself in the past, how does she find the exact moment to call herself before the mom and the police arrives at the house? Because if both timeline are continually progressing in the past and the present, won't that moment in the past also have passed? And my conundrum here is how this ending is a miss because it doesn't validate the space time continuum sequence of events here. Either leave the ending as is with the mom being alive and Young-sook dead or mom dead because Young-sook never died.
Things need to add up even if we're left with a cliff hanger but unfortunately this cliff hanger ended up making no sense.