What do I do with this unused iPhone 5s?

I'm wanting to sell this iPhone 5s but I'm having issues because it was put on a 2 year contract by my now ex boyfriend in May 2014, and I've been trying to call at&t to see if he removed the phone off the contract but they won't let me into the account without the last 4 of his social. I'm not in contact with my ex at all nor do I plan to be. What are my options as far as this phone? Right now the phone is unusable because I'm pretty sure he is data blocking me so I can't use the phones data or texting. I don't want to sell the phone while it is still in contract. My ex is an idiot when it comes to managing finances and I'm afraid of the phone being automatically put in another contract by AT&T without my consent. The phone was a birthday gift from him.

Most likely he shut the phone off and paid off whatever he owed on it when you broke up. So either it's dead because he cut it off completely or he could unlock it for sale if he didn't do that. But why would he want you to make money on the phone after he paid for it. Probably nothing you can do with it.