Elevator
(2008)

Two older teens, a boy and a girl, looking for a place where they can get away and be alone, go to an abandoned factory on the outskirts of town.  They get trapped in the cargo elevator between floors.  No one can hear them yell.  Multiple efforts to force their way out fail. They have a cell phone, but there is no signal.  

After six days without food and water, they are desperate.  There is a small air vent, too small for a person to fit.  The boy has the idea of writing a text then throwing the phone out through the vent.  If it lands without breaking, it will still send the text once it gets a signal.  He manages to land the phone on a blanket, so they see it is not broken.  

The movie fades to the closing credits as they talk, waiting for help to come.

After the credits, we see the phone showing no signal, and the text did not go out.  Presumably, they will die stuck in there.

Thanks Joseph C!