Almost EVERYONE has a cell phone these days - even the poorest of the poor, so anyone asking to use your cell phone doesn't seem like a trust-worthy individual. When you do a "good deed" for someone, there's a fine line between what infringes on YOUR safety and what is legitimately good from the bottom of your heart.
If there's an old lady who needs help carrying grocery bags, that's a good deed that doesn't cost you your own safety and there's nothing owed in return. Some small favors don't mean you risk your own losses.
I've helped random strangers when they've wanted photos of themselves taken -and they trusted me with their phone that I wouldn't take it - but they ASKED me to and I did it out of kindness, not expecting anything.
You can just tell someone like this "I don't lend my phone to anyone, it's my company phone and they monitor all of my calls."
Also, who cares if this stranger got mad at you - definitely NOT your fault she doesn't have a means of communication!