Seems it wasn’t that cell phone service was shut down, it was that it promptly overloaded by so many calls. Concentrate that many people in that small a space and give them all a reason to start making calls at once, yeah the system collapses. Hysterical callers may have saved lives by swamping the cell towers.
Maybe because the cell phone service went down, is the reason the rest of the bombs didn’t go off. That is, if they were being set off by cell phone.
This was my first thought.
you are correct, assuming the devices relied on cell phones for detonation (why not timers?)