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 dial their own number to get yours (assuming your number isn't private.)

I think I just invented a new, somewhat forceful and creepy, pick-up move.
They could dial a [USSD](http://en.wikipedia.org/wiki/Unstructured_Supplementary_Service_Data) to get supplimental information about you or your device. Some UUSD codes have been documented to have the capability of doing a factory reset on your phone. [Source](http://www.engadget.com/2012/09/25/dirty-ussd-code-samsung-hack-wipe/)