I bought an extra phone accidentally on Verizon (at full retail with no sim) and my friend who got his preorder in late on AT&T wants to buy it from me. Can he just put his SIM into the phone and it'll work or is it not compatible for some reason?
Since the 6 hasn't been released yet, I don't think anyone can confirm. That said, I have a Verizon iPhone 5c and it works just fine with AT&T and T-Mobile (even LTE).