And once they have snagged the code info from the cell phone, they can "switch it on" as a transmitter at any time covertly, so simply turning it off means nothing.
They use phase difference math to determine location, it's much more accurate than triangulation. With only one tower, it's not as accurate as they'd like, because it's depending on "RDF-like" technique (i.e., which antenna(s) on the tower get your signal the strongest) but it's still fairly accurate.
As to the remote-turn-on deal, I don't buy it as possible except for some very low drain digital phones, and "rigged" phones (i.e., it's possible to blow a palie terr's head up with a cell phone, but not with any cell phone :) )
The problem is, to turn a phone on remotely, the phone's receiver must be receiving. Even the "economy" settings (which turn the receiver on and off every few seconds) will cause the battery to run down at a known rate. When the phone is off, the battery lasts as long as it would if it were detached from the phone. This is pretty convincing proof that the phone is really and truly "off" when it's turned off.
Some of the newer digital phones have such low drain that a battery will have a standby time measured in terms of several days. It's possible that even if it was in a "clandestine on" mode when putatively "off", people wouldn't notice, because their routine would be to recharge long before the battery had run flat.