Wall Of Text, thinking aloud (read: ramblings) AND no clear information. Read at your own risk.
We've been analyzing the voice mail all this time, but there's other source of information we forgot about: our detectives' clients. They framed our guys, and they are clearly a part of the Cabal. So...
- Poirot's client is Arthur Moore, "Assistant Professor of Mathematics" "at King’s College London". Seems to be an Englishman. Leslie Okogwu's case was given to him on 28th of September. Also, Les seemed to be scared of Moore's name.
- Sherlock's client is more suspicious. The case was presented on 30th of September. The client is a Dutchman, called Elias Stein (Steinitz, anyone?). Met Holmes at the Chershire Pub. Wore dark clothes (Holmes noted that it must have been bought a day before), gloves, shades. It was also very dark, and Stein had a huge orange beard. In short, his face was obscured well. He also had a napkin from Cafe Nero (that's where Raikes works).
- Now we're getting to the Cabal. Angelus has started looking for compatriots on (or after) 17th Sep (first letter), they got together after 21st (second letter) and, I assume, before 23rd (Joe started to receive more letters). I also assume that Joe lives in the US. Our detectives resided in London. If the Cabal had to move from some other country, they should have both had a clear reason to frame them, and a Xanatos Speed Chess master - orchestrating a perfect crime in a week is no trivial matter. The problem is that we don't know where the Cabal's members were at the moment.
- Now it gets fuzzy. The problem is that I now realize that Moore could disguise himself at Stein, and that means that only 1 member of the Cabal would have to be present in London. Even though they mentioned that Moore was an Englishman and Stein was a Dutchman, I think the accents could be faked (I'm pretty clueless when it comes to accents).
Ok. To be honest, it all falls apart. If we assume that the "killed for stumbling at the wrong place at the wrong time" guy is Phantom, that leaves 2 more people from the cover. And, well, it's more of a premonition than anything, but that guy with the glasses just doesn't seem to be the right person for the remaining threats with a man's voice.
I think the only way we can make some sense of it all is assuming there are more than 4 people in the Cabal.
Or something.
Sorry. Tl;dr: we need to factor the old information into the equation. We have 3 sources: blogs, voicemail, and, probably, the comic.