I learned so much at school, hacking crappy computers because I was bored. Boot disks in my backpack, hex editing the typing lesson saves, packing emulators and ROMs in one floppy at time and merging them back together (I even wrote a BASIC program for this because I didn't know that tools existed to compress and chunk large files). And just exploratory hacking for fun, writing scripts and tools and stuff just to see if I could.
Chromebooks are the opposite of that, we bought our daughter a Chromebook and on realizing that it was only a tablet with a keyboard it went back to the store. She has my old Linux desktop now and knows a lot more than her friends
Without compromised hardware even igniting a battery is pretty implausible (unless the phone was on charge, and obviously these weren't) as you'd need to basically short it out and this would be hard even with full bare metal access.
Pagers are famously hard to hack as well since all they do is display strings. And they aren't on the public net, they don't even have IP addresses as they communicate hub and spoke with a big slow RF transceiver.
Much more likely triggered by a message or long time fuse.