Since in most of the places I hike (except the Columbia River Gorge and the south side of Mt. Hood), I lose cell phone reception long before I get to the trailhead, I just turn the thing off and hide it in the car. If I have the phone on and I'm in a place with poor or no reception, the phone quickly runs down the battery trying to get a signal. It is true that with marginal reception, the towers can pick up a faint signal; they may or may not be able to triangulate it. Where there's no reception at all, there is no cell phone tower around to pick up the signal (which will very soon be zilch because your battery will be gone).

Even in the Columbia River Gorge, you often can't get a signal when you're down in the deep narrow canyons. Most trails quickly climb (!) up to where a signal is available, but not all of them. I do carry my phone (instead of my PLB) when I'm in the Gorge, though.