How do you SIM-unlock the SGH-I317 (AT&T Samsung Galaxy Note 2) on Jelly Bean 4.1.2? I have looked all over the internet, and a lot of the download links and information has gone stale, or is incorrect. I wanted to standardize it here for other people.
Calling AT&amp;T may get you an unlock code within 24 hours. You do not need to buy one, or follow this technical guide. 

But if all else fails, you can unlock via a downgrade to 4.1.1. How to do this is below. 

The most reputable source I found is here: XDA Developers AT&amp;T Samsung SGH-I317,
a BIG thank you to: Odin Guide 
and last steps are from After 4.1.1

Estimated time: 2 hours.

About SGH-I317

This is the code that identifies the AT&amp;T USA version of the Samsung Note II. Every carrier has a different firmware, so you would need to use the firmware for your carrier.

About Unlocking

Unlocking is not the same as rooting. Unlocking let's you use your phone with other SIM cards. Rooting let's you have more control over the internal software system of the device.

About Samsung Odin

A 3rd party software used to change the firmware of Samsung phones.

About 4.1.2 Jelly Bean

This is the Android version of the phone. It is an older version, but onl version 4.1.1 has a free unlock menu.

About Kies

A similar to iTunes for iOS, a proprietary phone syncing tool.

About I317UCALJ2

This is your baseband version for the 4.1.1 on AT&amp;T USA. A list of all the baseband versions of the modems are here: http://www.androidayos.com/samsung-galaxy-note-2-stock-firmware/

Requirements


Samsung Galaxy Note 2 USA AT&amp;T Locked 4.1.2 Jelly Bean 
Windows Computer


Downloads Needed


Exact version: Odin 3.07 (newer versions will not work): http://forum.xda-developers.com/attachment.php?attachmentid=1168421&amp;d=1341114315 (may also need the Note 2 USB driver: https://mega.co.nz/#!yVJnlCAL!W2TKtTloGVWQtFD0AON1DG1pgPJCEqAVxonqlSPeqOg )
4.1.1 Original firmware, sometmies called J2 Modem for your phone: AT&amp;T is here: http://d-h.st/wsW Full List here: http://www.androidayos.com/samsung-galaxy-note-2-stock-firmware/
Samsung Kies: http://www.samsung.com/ca/support/usefulsoftware/KIES/JSP
7-Zip: http://sourceforge.net/projects/sevenzip/


Steps to Unlock


Disclaimer: If your phone is not an SGH-I317 (see device settings) or if you don't follow the steps or your not careful, you may brick your phone. I take no responsibility, follow at your own risk:
Charge phone to full battery.
Backup important phone data.
Install Samsung Kies
Install Samsung USB driver.
Enable USB debugging from your phone’s settings. Go to Settings > Applications > Development, and tick the box that says ‘Enable USB Debugging’.
Ensure that KIES is closed and not running in the system tray! Kies will interrupt and doesn’t like Odin, it will block you from flashing.
Turn off phone completly. THEN while off: hold the Volume Down key, Home button and the Power ON/OFF button simultaneously until it says Downloading.. Do not turn off target!!”

Plug in USB cable from phone to computer.
Use Samsung Odin 3.07 (for SGH-I317) to downgrade the firmware to 4.1.1 I317UCALJ2
In Odin window – the ID:COM box will turn to blue color and indicates the COM port number and a word “Added” on a log box will prompt you that the device is already detected or recognized by Odin. Your COM port number might be different from mine. If you do not see a COM port number then there’s something missing or wrong

Wait for the word PASS!! ** takes about 7 minutes ** 
Troubleshooting: Phone should reboot. If it gets stuck at samsung logo ad-infinitum (give it at least 2 literal minutes, sometimes it needs to process for a bit), try to poweroff by holding power button. Worst case, remove battery. While off: Hold Volume Up, Power, and Home buttons and do: 
1) Wipe data / factory
2) Wipe cache partition
Dial this into your phone

*#1234#
Verify you have 4.1.1 now (I317UCALJ2) Settings &gt; About Device.
If this did not work, see this guide for troubleshooting:  http://www.androidayos.com/2013/04/07/unroot-and-restore-back-att-galaxy-note-2-sgh-i317-to-original-firmware-stock-rom/
Finally, Dial the following into your phone app.

*#197328640#
Select  [1] UMTS &gt; [1] Debug Screen &gt; [8] Phone Control &gt; [6] Network Lock &gt; Options [3]Perso SHA256 OFF &gt;
After choosing this option, wait about 30 seconds.
Then go back one step by pressing the Menu button and then select Back.
This will take you to  [6] Network Lock options
Choose [4] NW Lock NV Data INITIALLIZ
Wait for a minute then reboot your phone.
Insert a SIM Card from a non-AT&amp;T carrier