How to use phone dial tone as an interaction controller, and decode DTMF signals

The first article in 2020, accidentally picked an Old School topic. There was a section in 名探偵コナン 戦慄の楽譜フルスコア(Detective Conan:Full Score of Fear), released ten years ago. Konan was standing in the middle of the water. First, a world wave shot down the phone receiver on the shore, then closed his eyes and shouted loudly. 110 alarm calls were broadcast remotely. This time I will talk about how to use sound