He likely means to have the phone used as a remote trigger for explosives. Something like this...
“GSM Based Cell Phone Controlled Remote Switching Circuit”
http://homemadecircuitsandschematics.blogspot.com/2012/01/how-to-build-gsm-based-cell-phone.html
Even simpler....
A simple amplifier circuit hooked into the headset jack and set a rather loud ring tone.
You could even set it to trigger using specific number by setting the default ring tone to silent and use a specified contact number in the contacts list to be the loud ring tone...
No opening up a phone a soldering required. Just get a burner phone set a few options plug it in and no false calls can trigger it.
(Note: the above text is not a statement of intent. But merely an example of how technology can be abused)