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.
While detonating a bomb or EMP would be the most harmful, I think the following scenarios are much more likely to happen.

If you have a smartphone, it's very likely you have some kind of weather widget on the homescreen which tells the attacker what is your hometown.
Also, I yet have to see a smartphone without news widget, which again, tells the attacker what kind of person you are. Are you following a finance news? Sport? IT? The very same thing can be done by just looking at your installed apps. Do you have any games? Which ones? Do you have any kind of booking apps? Does this kind of app keeps any sort of history? Well, in short, within just few second attacker can make pretty reliable profile of you.
Do you have anykind of notebook app? What have you written in it?

The second threat is... Well, most likely you just gave him full access to all of your e-mail accounts and with a few presses on the screen, he could forward all of your mails to his account.

The last thing which I can remember are pranks. While this doesn't sound like anything danger, it can be really unpleasantly. The attacker could send to a random contact of opposite sex SMS - "Hey, I'm thinking about you...". Just imagine if this random contact is your wife's friend (now, the scenario of detonating nuclear weapon is not so scary, isn't it?). Or even more explicit message to a contact saved as your family member (e.g. Mother).
He could also update your Facebook status, leave a message on a Twitter or upload some of your private photos to a public service.