I'm not sure if it's a trope or not, but there should be almost nowhere on the surface of the planet that cannot get a cell phone signal. Watching the hoops writers have to jump through to justify why their protagonist can't just call for help when the ax-crazy psychopath is chopping down their door is getting old. Instead of taking away the phone or having chronically-forgetful characters never remembering to top off their batteries, let's see something more creative. :)