Not sure I follow here. After reading online the mega uses the micro sim card. You usually get a carrier specific card. So the one you bought from ATT won't work on a phone locked to Tmo. It's the phone that you need to be unlocked, not the SIM. That should always be locked to a carrier. Boost and Virgin mobile don't use sims, but CDMA. Which is completely different. You should get a sim card for whatever network you use. For example, I use Straight Talk and bought a sim from them. I have now bought 3 unlocked phones and have transferred the single sim between them as they have died/I've upgraded.