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.
With some phones it is possible to call system commands or even to lock the SIM card, just by visiting a prepared website. There was an article some time ago on [Heise security](http://www.h-online.com/security/news/item/Android-smartphones-USSD-calls-can-kill-SIM-cards-1719230.html) about this problem.