I am in Australia right now with my iPhone 6 under AT&T, and they refused to unlock my phone for international use. I am now using Optus with no problems.
First, I messed up my order and gave them the wrong number for my IMEI, and they fixed it for no additional charge. The unlock codes took a while to arrive (usually around 3-4 business days), but I didn't even need to connect to iTunes once I installed the new SIM.
I will definitely recommend this company to my friends traveling over-seas.