Initial concept (phone call between different times) has already been done, but the way it's used is interesting there.

The story is ok, It looks nice, the main actresses are good, and through all the first part and even the first timeline changes it's pretty well done.

Then we see Young-Sook turn into [spoiler]a crazy killer[/spoiler], and a cat and mouse game, going both ways, one using her actions in the past, the other her knowledge of future events to get the advantage. That part is still enjoyable with some good ideas.

Sadly the way the timeline changes are handled makes no sense at all. Most important issue is that she does not seem to get any memories from the new timeline, so she's just here, her whole life has changed, but she doesn't know how. The scars appear and disappear though, with the pain at the moment the timeline changes, so... why the scars and absolutely nothing else ?

She also reappears right where she was, which was plausible the first times, but not really when she gets dropped in the middle of a tunnel without a car. So, what happens to the her that was living in the new timeline ? She just disappears and gets teleported there ? Very weird. That could fit with the no memories part, she just gets projected into that new timeline from her own, but in this case it does not fit with the scars appearing.

Also it looks like only killing someone has an impact and initiate a timeline change.

Too bad the whole thing has not really been thought through, this could have been a real gem.