Bill, that app won't work if, as is quite common out here in the west, the last "ping" from my cell phone was while I was still on the road, at least an hour's drive from the trailhead! That's a common situation most places where I backpack. Unless I'm parked in an area where there's a high chance of a car burglary, my cell phone stays in my car.

In low population areas, there is no financial incentive to set up cell phone towers, except close to towns where there are enough subscribers to make it pay. In wilderness areas, of course, cell phone towers, to say nothing of the technology needed to put them there, are illegal.