I watched the series a few years ago and I can't really remember what happened. Does anybody remember if the reason for the existence/why the walkie talkie was working ever explained?

Thanks for answering in advance haha~ I won't be able to rewatch the series XD

Everything before 2000 for Jaehan was an alternative Haeyeong, on the psychiatric hospital is the first contact with our current Haeyeong and as he helps Jaehan the former present is changed, all off their connections relied on Jaehan's death on August 3rd, 2000 at 11:23. Since the message that tipped Jaehan about the hospital was Haeyeong's last, the following message would have been Haeyeong's first, but the past was changed and no more radio transmission.