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.