I don't remember there being a open ending. Well i did watch it a while ago. Must of forgotten as usual. Can you…
The walkie talkie flickered at the end and we saw Lee Jae-Han alive in the hospital. To put it in short it was an open ending because it seemed like there were different timelines activated, and only those who knew about the walkie talkie are conscious of this fact. Just a reminder that the latest timeline in the ending of "Signal" was that Jae Han remained alive instead of dying and leaving the walkie talkie for Hae Young to find. This because he got help from Hae Young in "today" time. Hence the walkie talkie flickered again and transmission started and we see Jae Han in the hospital looking at it (the absolute ending of the series) but we don't know for what exact reason, hence the "open ending". I don't know if I explained it well but this is as much as I remember from it, it's been a few years!
To put it in short it was an open ending because it seemed like there were different timelines activated, and only those who knew about the walkie talkie are conscious of this fact. Just a reminder that the latest timeline in the ending of "Signal" was that Jae Han remained alive instead of dying and leaving the walkie talkie for Hae Young to find. This because he got help from Hae Young in "today" time. Hence the walkie talkie flickered again and transmission started and we see Jae Han in the hospital looking at it (the absolute ending of the series) but we don't know for what exact reason, hence the "open ending".
I don't know if I explained it well but this is as much as I remember from it, it's been a few years!