Charlie speculated this must be some sort of code that they had already agreed on between themselves. This way, only themselves would understand what these different numbers meant so that they could achieve basic communication encryption. So, even if anyone else were to get their hands on Aota’s cell phone, the other party would have no idea what they were talking about anyway.
However, Charlie had a feeling that the real meaning behind the 07 sent by Masatetsu was to ask Aota about the situati