A plausible ending theory (that I've thought of), is that Lee Jae Han will die in the future and his walkie-talkie will end up back in Park Hae Young's hands again.

You know when Cha Soo Hyun got the message when she was not allowed to go to the hospital where Lee Jae Han was in the drama on February 5th? 010-469-3600 is the phone number of Park Hae Young, And he tries to contact Soo Hyun in the future to forbid her (but actually the forbid is an order, like Jae Han did when Soo Hyun told him not to go to the Mental Hospital Seonil, he still went there.)

It's still a theory conspiracy, but i think this is was the most that is makes sense Ig. Anyway, correct me if i wrong (⁠T⁠T⁠) May be the message that KarinAllen said is true? : we don't know

3 years later Karin..... I just finished Signal with a big question mark on my head but I like your explanation and I'm going with that! Just in time for the announcement of the release of Signal 2!