I've never had any issues talking to AT&T's international support, saying I'm planning on traveling overseas and requesting an unlock code. No charge.
I am long time AT&T customer and never been successful with AT&T's international support. I ended up buying codes on the net.
When I received my S4 a year ago it clearly stated in their website that S4 would not be received the sim-unlock code from AT&T.
I received the S4 with 4.4.2 rom and MDL baseband/modem. I rooted the phone and then flashed the custom rom (Clear Rom 1.5). After that I tried the hidden features that posted on the net to unlock the sim. It did not work. The phone has to be on stock rom that I read later.
I end up buying the code on the net and it worked for me. When I bought the code they gave me several codes to test it. Sim was unlocked in the first code that I entered.
