What happens if I keep a phone that I find and demand money from the owner to return it to them?

I found someones phone the other day. My mom said to tell the person to send me $20 so that I would send the phone back. There was a 'home' number on the phone so I called it and got in touch with the owner of the phone. I thought I'd be nice so I just let the person know I had her phone so that she could come get it. She said she lived too far away so she wouldn't be able to come get it then but shed call back and let me know when she'd pick it up. I said fine. She never called then I called her back 2 weeks later so she said she's send me a prepaid envelope so that I could mail it to her. I feel like I should have asked her to send me some money if she wanted her phone back because I did not like her condescending tone of voice at all. She is lawyer and she has my address. What could happen if I just kept the phone out of spite or demanded money to get it back?