Please see the SIM Unlock Policy | Unlock Your Mobile Wireless Device | T-Mobile page for details regarding this procedure. You will need to also Contact T-Mobile in order for them to process your unlock request.
Let me know if you have questions/comments!
My iphone 6
iCLoud status*: CLEAN Blacklist status**: CLEAN
You need to meet the requirements in that article to be eligible to get the phone unlocked by Tmobile.