YandereDev has metioned before about the Yazuka in one of his videos. During the video, there was a fanart of the Yazuka and Osana who was tied up and gagged. Prehaps her phone call has something to do with this?
I don't think it was in a video. But do you remember when you could go into the game's files and add Osana into the game? Well, if you do that inn a previous build of the game, you can wait until osana gets a call
i think that it's info chan. osana calls the person calling her a creep, which info chan is. second, info chan has something against osana, which is revealed in the intro texting. third, info chan is one of the few students that knows about osana's crush on senpai.