It was obvious the innkeeper was the culprit (having an isolated cabin, has knowledge that there will be no signal during the blizzard, presumably was out to cut the phone lines when he had a one on one chat with Arisu, but the fact that the bang rang out when he was outdoors did throw me off).
But I guess this is the end of the story, seeing how she is practically comatose now unless the fortune teller suddenly shows up and does a deus ex machina.