Never let a stranger borrow your phone to make a call. It doesn't matter how much in need they appear to be or how nice a person you are. Chances are they will take that fancy smartphone of yours and run with it.
Also, no matter where you are, be careful when you do take out that fancy smartphone. I was walking toward Penn Station to get home one night, saw someone with their Iphone out checking something on it. All the while someone who was walking by tried to (in one motion) pull the iphone out of the guys hands. Lucky for the owner of the Iphone that the guy didn't succeed.
And,ghostlight you're an idiot. Just because you're listening to music rather than someone begging for money on the subway, does not automatically equate you loosing your phone. You can still listen to music and be aware of what's going on around you. Hell, the only way you're going to loose your fancy smartphone is if you don't keep it in a pocket or close to you if you're using it on the subway. If you wave it around for all to see, then yes, you're going to loose it. However, if you're discrete with it then chances are likely that you're going to walk out with your phone.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll