Seems like that would entail something in the phone that could see if it's connection to the base stations had unexpectedly fallen to almost zero. Assuming this is how being jammed works. This is probably quite hard to do on any non-sdk'd phone, so it might not have come up yet. Iphone prices probably need to drop a bit more...
Cleary a dedicated radio detonator could be programmed to work like this, but then we are talking improvised here.
Radio detonators are not so easy to improvise. So you end up buying off the shelf parts. But since there are many fewer radio controlled devices, this means your signal is a lot easier to trace. The nice thing about cell phones, is that there are a lot of them. And activation can be on a ring. How do you distinguish a hostile call before it has even been answered?
You just need to temporarily setup the network to block incoming calls to the phones in the patrolled area. This does not mean the 'signal' will drop to zero at any time, from the phone point of view. The phone will have absolutely no way to tell a blocked call from a lack of calls.
Cleary a dedicated radio detonator could be programmed to work like this, but then we are talking improvised here.