It is common practice to call twice in a row for emergencies. So if a random number calls twice in a row, then you should pick up.

That's what I tell my friends and family, but I don't think it's all that commonly known among the people I know :/

or if that call is really from your known ones then they will surely leave a voice mail

Another thing is that in the US junk calls more often than not come from out of state.

Except that cell phone numbers tend to tell you where somebody lived 5 years ago, as opposed to where they live now. This makes it quite hard to screen based on an out-of-state area code.