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: 
with a USB rubber on android the pin password could be hacked (brute forced)
<http://hakshop.myshopify.com/products/usb-rubber-ducky>

they could then create a backup of your device, they could analyse already created backups, they could download all of your saved data, media etc and use this to further penetrate other areas of interest. 

<https://santoku-linux.com> 

they could run the phone through a smartphone pentest framework, and infect it for botnet purposes. 

<http://georgiaweidman.com/wordpress/>

they could hawk it for cash 

<http://www.cashamerica.com/>

**on a side note, if your lucky they could;**

1. correct any duplicate contacts you have
2. organize your mobile media
3. finishing posting your facebook post you left open
4. beat that hard level on Angry birds for you
5. Call your most called numbers to return your phone
6. Call the carrier to report it lost/stolen
7. etc
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.