My phone is locked due to not fulfilling contract! Can I unlock it somehow?

I got a sony xperia x10 in March I paid 200 dollars with an upgrade with ATT. My dad decided to go with a much cheaper provider and since the contract wasn't fulfilled my phone is locked! Is there a way to unlock it so I can sell it or at least use the wifi and apps on it. I can't get ATT myself so I just went with metro pcs