Well, I decided that paying $130 a month for service with AT&T (and overage charges!) was to much and I was tired of the constant fees being added. Time to move to T-Mobile I thought, especially after seeing the latest in carrier event which includes no DTA charge if you use Pandora and a few other music streaming services AND of course no overage fees, ever! So now I have unlimited everything for $80 a month. So here was my problem. My wife and I have Windows Phones (Lumia 1020 & 1520). My wife refuses to use any other phone now (she used to be a diehard iPhone user!) Anyway, T-Mobile doesn't sell either phone and my 1520 is an AT&T exclusive model. But... Got everything to work without issue. If you want to move to T-Mobile and have a Lumia, here are your steps. Warning: this may require you to have been on your current AT&T plan for at ll east 6 months. I know I was. 1. Go to T-Mobile with any phone other than the Lumia (but uses the same type of Sim, whether it be micro or nano) and have them set you up with a plan and port your current number over from AT&T. This will cancel your contract with AT&T allowing you to request an unlock code for your Lumia. See my warning above regarding having at least 6 months on current contract. 2. Go here and request unlock code from AT&T. Note: My Lumia was paid in full, but my wife's was not. We both got unlock codes with 24 hours of request. Link: https://www.att.com/deviceunlock/client/en_US/ 3. Once you receive the email with your unlock code from AT&T, insert the T-Mo Sim into your Lumia and turn your device on. The Sim won't be recognized and you'll be requested to input your 20 digit unlock code. Once done wait up to 10 minutes and your Lumia will now be on T-Mobile. If you are a 1020 or 520 owner, you are done. If you are an AT&T 1520 (RM-940) owner, read on: HOW TO: Requirements (Pre-requisites) AT&T 1520 Phone T-Mobile NanoSIM Foreign NanoSIM (I used an Orange SIM from the UK) Access Point App, installed on the phone Link: http://www.windowsphone.com/en-us/store/app/access-point/ce3895c7-01d0-4daf-a4c3-25c10463942d Step-by-Step for getting ATT 1520 working on T-Mobile: Part I : Use Access Point and the foreign SIM to configure tethering settings Insert foreign SIM card into phone and boot it up. Goto Settings->Access Point, launch the app, and verify that it applies the foreign settings (it should read 'active' underneath the Mobile Operator settings being used) Reboot the phone to ensure the settings 'stick' Shut down the phone. Remove the SIM card. Part II: Uninstall Access Point to keep the tethering settings. (If you don't do this and boot with the T-Mobile SIM, Access Point may apply a different set of tethering settings.) Boot the phone Go to settings->Time & Date Turn off Automatic Date, set the year to 2113 (or something ridiculous. You may get requests to update gmail and outlook... Ignore these) Go to settings->Access Point. When you launch the app, it will request to uninstall because it has expired. Uninstall it. Restore the time & date settings, shut down the phone. Part III: Boot with the T-Mobile SIM and configure APN/MMS settings Insert the T-Mobile SIM card, boot phone Goto Settings->Cellular+SIM Configure & save internet settings with THIS info: Under 'edit Internet APN' ONLY input the following info APN: fast.tmobile.com Nothing else. Every other block should be left blank. Hit save. Goto 'edit MMS APN' and only input the info for the following data: APN: fast.tmobile.com MMSURL: http://mms.msg.eng.t-mobile.com/mms/wapenc Port: 80 Leave all other blocks blank. Hit save. Now, send yourself a picture MMS to apply the new MMS and data settings (this is important). Go to Settings->internet sharing, turn on sharing. At this point, you should be able to successfully do everything that a device should be able to do. Send picture messages. Share internet. I now have unlocked Lumia working on T-Mobiles network. I just don't have LTE on T-Mobile, because the band is missing on my version of the 1520. But my 4G speeds on T-Mobile are faster than my LTE speeds on ATT! Go figure.