I just bought a samsung galaxy s4 that was used on st. but my carrier is att can i use it?

[AC Question]

Do I need to go to an att store and what does unlocked and locked mean?

 

[srkmagnus]

Do I need to go to an att store and what does unlocked and locked mean?

Was the previous owner using it on AT&Ts network while on Straight Talk? Or T-Mobile's? Because that will make a difference in terms of the type of data speeds you are able to get. Regardless, the device should be unlocked if it was being used on ST. And yes, it will work with AT&T but the model S4 may restrict your data speeds. If you can provide us with the model number (system settings > about phone) we can try assist with confirm if you will get full useability on AT&Ts network.