Typically, Sprint devices will not work on AT&T's network, but I'm not sure if that has changed.
Gotta check the bands. If it's unlocked, it may work. I know for the 7 it will bc it's the same model as the Verizon one and I'm using a Verizon one on ATT.
Did you have any problems with MMS on AT&T using the Verizon version?