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: 
A few scams I've seen making the rounds:

* Use it to dial a premium rate number owned by the group. In the UK, 09xx numbers can cost up to £1.50 per minute, and most 09xx providers charge around 33%, so a five minute call syphons £5 into the group's hands. If you're a good social engineer, you might only have a 10 minute gap between calls as you wander around a busy high-street, so that's £15 an hour (tax free!) - almost triple minimum wage.
* Use it to send premium rate texts. The regulations on there are tighter, but if you can get a premium rate SMS number set up, you can charge up to £10 per text. A scammer would typically see between £5 and £7 of that, after the provider takes a cut. It's also possible to set up a recurring cost, where the provider sends you messages every day and charges you up to £2.50 for each one. By law the service provider must automatically cancel it if they send a text sayin STOP, but every extra message you send gains you money.
* Set up an app in the app store, then buy it on peoples' phones. This can be very expensive for the victim, since apps can be priced very high - some up to £80. In-app purchases also work. This is precisely why you should be prompted for your password on *every* app purchase and in-app purchase, but not all phones do so!
* Install a malicious app, such as the mobile Zeus trojan. This can then be used to steal banking credentials and email accounts. This seems to be gaining popularity on Android phones.
Some mobile networks in the world allow users to transfer prepaid balances from one account to another. Alternatively, they might send some sort of incriminating SMS from your phone which may cause you issues with law enforcement officer.