In the movie world there appears to be two major rules regarding phones:
1. Just hang up without saying goodbye
Part of this is presumably so screenwriters can quickly get to the next scene without being slowed down by what would actually happen in real life (i.e. awkward goodbyes or ‘just another thing’).
2. Go to a place where there is no signal
This is a tool which immediately adds tension, as characters in a remote place lose the safety net option of calling the cops (although aren’t cops always late in the movies anyway?). The fact that it has been done to death has made it something of a cliche in recent years.
Got any others?