Hi, I'm looking to get t-mobile service. Does it make much of a difference if I buy an at&t s4 phone and unlock it, vs. using a t-mobile s4? I unlocked one at&t phone and it can make calls, but not use data on t-mobile. what gives? Can I use the hot spot feature on the at&t phone? Can I put a t-mobile ROM on the at&t phone?