Starting December 31, 2014, it will no longer be possible to unlock Windows Phone 7.x devices for app testing. We recommend that you unlock your Windows Phone 7.x devices prior to December 31, 2014 to continue testing with those devices for another 24 month.
After December 31, 2014, no Windows Phone 7.x unlock capabilities will be available. This means, for those of you who have not unlocked your Windows Phone 7.x device, testing of apps directly from Visual Studio on that device will no longer be possible. However, app testing can still be done on other unlocked phones, or via the Windows Phone Emulator, or by submitting beta apps through the Windows Dev Center.
Please note that this change will not impact apps available in the Windows Phone Store, nor will it impact customers with Windows Phone 7.x devices. Additionally, if you have apps that only have a Windows Phone 7.x package, now is a good time to update these apps to Windows Phone 8 or 8.1.