Wow! This was painless. I bought my phone off of Craigslist and just chatted with international support. See below:
Teresa: Hello! My name is Teresa Garcia and I am an International Care Representative. I’m reading your inquiry and I will be right with you.
Teresa: I will be happy to assist you with your international concern.
M M: Thanks.
Teresa: Thank you for waiting. I'll be with you in just a moment.
Teresa: May I please have the IMEI number of the device? To retrieve the IMEI number, please dial *#06#.
M M: 353*********365
Teresa: The unlock code is 963****************0, I will provide instructions.
M M: Great
Teresa: 1 Remove the AT&T SIM card. 2 Switch the phone off. 3 Insert a SIM card from another wireless carrier into the phone. 4 Switch the phone on and enter the SIM card with PIN code, if requested. 5 Wait for the SIM unlock user interface to open. 6 Enter the unlock code (20 digits). 7 Press Enter.
Teresa: You have 3 tries to unlock the device.
That was it!