Palestinians stopped using cell phones because they could be geolocated by triangulation of cell towers. They began using pagers.
Prerequisite knowledge: If a lithium ion battery is exposed to air it'll burn hot and fast. If it's severely overcharged the protective case can be breached. But, it wouldn't take much to breach the case on purpose.
Israel surreptitiously sold the Palestinians pagers rigged to use the lithium ion battery as an improvised explosive device. Then, they detonated them all at once.
Editorial:
It's pretty fucked. But, they're killing much for indiscriminately already, targeting hospitals and schools. The OP exaggerates only the escalation to chemical weapons. Israel is a tool of the US's most successful proxy war.