OK, this process has been hell, but I'm gonna try and consolidate everything I've worked out so far in one place. This all applies only to the T-Mobile variant (ie. it comes preloaded with all the T-Mobile bloatware) of the N200 on the T-Mobile...
If you have the unlocked variant, you can skip the SIM unlocking steps, and no unlock code is required.
0. backup all user data as it will be lost upon unlock. Consider updating your software to the latest before unlocking.
1. Enable developer menu / mode on phone
2. Allow Google Play Services to connect to the internet for blacklist check on phone
3. check the "allow oem unlocking" option in developer mode
4. check "USB debugging" option in developer mode.
5. install drivers and ADB / fastboot software on a computer
6. connect phone in fastboot mode to computer
7. fastboot oem unlock (this will wipe your data)
Make sure you give the phone plenty of time after an update before you unlock it, to avoid the wipe bricking your phone.