I bought an AT&T GSM iPhone 6s Plus full price via the Apple Store App. It will arrive unlocked and without a sim card. I have an existing iPhone 6 Plus which I will backup with encryption. I was told by AT&T support that I put the sim card into the new iPhone after I restore the new iPhone from a backup. I wanted to reach out to find out if that's indeed what I should do. I had thought that I should put the sim card in before backup, but I guess not.