You don't need it to be unlocked if you're going from an AT&T phone to another AT&T phone. Just get the iPhone that is programmed for AT&T so you'll have 4g data. The only other thing to worry about is the size of the sim card - the 5c uses a nano-SIM

I have a basic cell phone(LG Expression) using ATT cell phone service. If I buy an unlocked Apple IPhone 5c and move the SIM card from my old basic phone into the Apple, will it work on ATT cell phone service? Are there any pitfalls to be aware of?