Well, I went through the process of buying the Nokia Lumia 520 direct from Microsoft (and it came in the same packaging you would find at Walmart or Target or Best Buy) and wanted to use it on any GSM network, so first I tried using an online place that 'claimed' they could get me an unlock code for $6.95. After 3 days of them trying, they claimed it was not possible and refunded me the money. Step 2 was to go through AT&T's online 'unlock request' form found here: https://www.att.com/deviceunlock/client/en_US/ I am in good standing with AT&T, have been with them for over a decade (since the old NEXTEL days), but I no longer have a '2 year commitment' contract with them, since I buy my phones off contract. I requested an unlock code for my 520, figuring what they hell. I have several T-Mobile pay as you go sim cards and want to use my Windows Phone without having to swap out the sim from my Nexus 5. By the way, it says you have to wait 6 months from purchase to request an unlock code. I have only had the phone a few weeks. I got the unlock code today. I inserted the T-Mobile sim, turned the phone on and got the prompt to enter my unlock code. **Note: You get 10 tries total. After that, the phone locks to the network and can never be unlocked after that. I entered the code properly and boom... now I have a Lumia 520 running on T-Mobiles network. I called *233 and added $15 to my pay as you go plan on T-Mobile. No problems, no fuss. Easy as pie. I don't have LTE where I live, but I do have 4G and I am getting a nice strong signal. I suggest that if you want to get off AT&T and go to T-Mobile or another GSM network and hate being 'locked' in to anything, then try requesting an unlock code. As you can see from the pics below, I have a good signal, the phone still believes it is a 520 from AT&T, but the 3rd pic shows my active network as T-Mobile.