I'm a current AT&T customer under contract with a locked iPhone 6. I plan to leave for T-Mobile in the spring. I'm thinking of purchasing a Sim-Free (unlocked) iPhone 6s Plus, which would make the transition to T-Mobile in a few months easy. However, I am a bit concerned my AT&T SIM will result in a locking the new iPhone 6s Plus. Does anyone know if that's the case? I have pre-paid T-Mobile SIMs I've been testing with. Should I insert those into the device for the activation process? Or maybe the Sim-Free models don't require activation at all? Any help would be appreciated.