Question: A stranger walks up to you on the street. They say they lost their phone and need to make a phone call (has happened to me twice, and maybe to you). What's the worst a phone call could do?

Let's assume they don't run, don't plug any devices into the phone, they just dial a number and do whatever, and hang up.

Answer: 
They could use it to send the detonation signal to that nuclear weapon they've secreted in a warehouse in Manhattan. That's pretty much the worst-case scenario.
Rule 3: [If a bad guy has unrestricted physical access to your computer, it's not your computer anymore.](http://technet.microsoft.com/library/cc722487.aspx#EIAA)