The happiest interpretation is that she was the unlisted number calling him, except it means the moment he finally resolved to move on with his life she ripped that scab open fresh again, but at least she didn't die of her mystery bullshit. The worst is Takt because fuck that guy. It's possible the last tank will give something along with whatever lewds the other has planned, you can't just dangle that phone call at the end and be all 'lol the end fuck you.' Well you can, but that's just a surefire way to piss a bunch of already pissed off readers even more. That twitter link up there has even more "what was that ending" replies now. I don't understand the people thinking it will be Shizuku/Takt content. He was practically forcing himself to do what little he did and she has zero reason to reciprocate now that the guy finally came clear, unless it's just more what-ifs/delusions