I have a big, yellow RM-938 1520.3. This is one of the Factory Unlocked versions.
I was on T-Mobile... Everything worked fine except no T-Mo Apps like WiFi Calling or any other features reserved for T-Mo branded phones. No unlock code needed.
Just yesterday I put it on Straight Talk with an AT&T compatible SIM card. Everything works fine. No unlock code needed.
http://forums.windowscentral.com/t-m...enta-lass.html
If this is the one you are looking at, it will work on any SIM based US carrier or airtime reseller. If it is an AT&T 1520 (RM-940) that has been unlocked from the carrier, it will still work with Straight Talk or any AT&T reseller and needs just a slight mod to be 100% compatible with T-Mo towers (instead of 95%).
http://forums.windowscentral.com/nok...-4-1700-a.html
Check out the 1520 forum here to see how much users LOVE this phone.
Nokia Lumia 1520 - Windows Central Forums
Consider joining Windows Central so you can participate in threads and be a part of this great community.
Hope some of that helps.