Apple does not allow carriers to install custom software or brand the OS. Apple also removed Carrier IQ from iOS years ago. A phone bought from a carrier should be identical to the same carrier version bought from Apple.
[doublepost=1458700058][/doublepost]You can buy an AT&T phone from Apple at full price. It will be unlocked and come with an AT&T SIM pre-installed. Depending how old your existing SIM is, it might be a good idea to get a new one since they are sometimes needed to unlock new carrier functionality.