AT&T should unlock the Lumia 900 for free, but they do have a couple of restrictions. If you're a business customer, you can call 800-331-0500, and after verifying your eligibility, they'll give you the code on the spot, with an email follow up. If you're a consumer, you can visit their unlock request website at http://www.att.com/deviceunlock
. The general requirements list the circumstances that they will unlock your device.
Current AT&T customers may request five device unlock requests per year. Former customers may request up to five device unlocks per former account.
Hope this helps.