I don’t think Johnny Jihad has to get that complicated.
He simply does not turn on the phone until it is in place and he is ready to leave or bury it. (or however it is placed).
Slightly different idea then...
Cell phones are set up to lock on to the strongest signal. This will generally be the closest cell tower, or the one with the best line of sight. Added on top of that is a preference for “home” system towers. eg. a T-Mobile phone would prefer a T-Mobile tower, etc.
Suppose your convoy is led by, and accompanied by, several vehicles with mobile cell towers. Now, they’re not really hooked into the phone grid. The on-board computer is just smart enough to do the exchange/handshake with cell phones. Thus, your antenna(s) “capture” all the phones nearby, over-riding the cell towers by virtue of a stronger signal.
The upshot is, for anyone nearby - dropped call as you approach. While passing - no calls in or out since your mobile cell towers don’t go anywhere. Result - no incoming calls, no IED detonations...
Just another thought. If they’re using technology, we ought to be able to use it against them...