So, I purchase an AT&T branded Surface 3 LTE and am now switching carriers. When I submitted my IMEI number for a SIM unlock, it said the number wasn't found in their system. When I called AT&T directly, they stated that they don't SIM lock their tablets. Inserting a t-mobile SIM will bring up a carrier unlock window however. AT&T said that I would need to contact Microsoft. I've contacted Microsoft now and they tell me that there is nothing they can do for me - that AT&T will need to unlock it. However AT&T still maintains that they don't lock tablets so there is nothing they can do. Does anyone have a thought on where to go from here?