Comment by bluescrn
If the footage on Twitter is legit, then there was a small detonation, with a bang, not a burning battery with a 'whoosh' and flames. Which indicates that the internals of the pager had been replaced with something rather more explosive than a lithium battery.
Lithium ion batteries in devices are sandwiched layers enclosed in a kind of 'pouch', right? So what if you manufactured one of these that looked identical to the normal battery, but only had half a battery inside, and the rest of it was plastic explosive. Maybe put a tiny chip in there that, when a particular pattern of current draw happens, fires a detonator. Then, some firmware hack in the device proper that responds to some event and actuates that current draw pattern. It wouldn't even look suspicious if you opened it up.