What happens if you don't return a phone to Verizon?

[Slack3r]

Well, I've had my TP2 for a while now, and I love it, but I've had a problem with dead/stuck pixels like I've mentioned in a few posts. So I called up Verizon, and they sent me a replacement, and temporarily billed me the cost of a new phone until I sent back the old one.

Now, my girlfriend has seen my phone, and she absolutely loves it. She doesn't care about dead pixels, which brings me to my question: What happens if I didn't return my old phone to Verizon, and gave it to her?

I don't want to do anything illegal or against Verizon's TOS, but as I've already been billed for the phone, I was just wondering if this was possible. Any thoughts? My only concern is, since they billed me for the price I paid on contract, would they charge me the full amount ($489) if I didn't return it, or just leave it at that?