In short, I really liked watching all the episodes and it kept me wanting and waiting for more. Did a great job in justifying every second of the screentime IMO. Everything (storyline + theme + OST + cast + acting + genre + setting) are stitched well together. I have not read the novel so cannot tell how good it was in adaptation but as a watcher/viewer, it was very entertaining.

If you are still asking, "Why?", here's in short:

1. Its perfect for thriller-rom fans.

2. The plot has many twists and leads have great chemistry.

3. Production and writing is great with details.

4. Pace is perfect.

PS: Here's a link to an open list of all details/gems found during the drama

https://mydramalist.com/discussions/general-discussion/133588-when-the-phone-rings-wtpr-details-gems?pid=3206600&page=1#p3206600