When you purchase a T-Mobile branded OP 6T the device will be SIM and Bootloader locked. Many great people here on XDA found ways around the SIM and bootloader lock. I am simply compiling it all here in one simple thread to be a resource for us T-Mobile users. In this guide you will be directed to the proper resources to SIM and bootloader unlock your T-Mobile 6T and return to T-Mobile software fully unlocked. It doesnt matter if your device is paid off or not. You are bypassing the 40 day wait for a paid off device to SIM unlock!
This is a high level overview for someone that wants quick info and links. All of the info is on XDA but scattered in various threads. I found bits and pieces here and there in those threads so I decided to make a dedicated T-Mobile thread for any T-Mobile user that needs help with the complete process in one simple thread.
So here goes...
Step 1: Convert device to international ROM and unlock the bootloader. Thanks to @AnonymousTipster for this thread! Follow the steps here: T-Mobile 6T to International Conversion (WITHOUT unlocked bootloader/SIM unlock!) NOTE: You MUST use version 9.0.11 to unlock the bootloader! Do not use newer versions because they closed whatever loophole that allows us to unlock. After converting to the International version go to Developer options and select OEM Unlocking and turn it on. Now reboot to bootloader and issue the command "fastboot oem unlock" to unlock the bootloader. You are now on the international ROM with your bootloader unlocked.
Step 2: Root and Recovery: Thanks to @mauronofrio for this thread! Now you can root and install recovery on your device following the steps here: [RECOVERY][3.3.0-2][fajita]Official/Unofficial TWRP recovery for OnePlus 6T (Stable).
Step 3: Unlock the SIM . Thanks to @nika_bego for this thread. Follow the steps here: Guide SIM UNLOCK T-MOBILE version all type of IMEI supported.
---NOTE: At this point you can call it quits and keep device as is which is now both bootloader and SIM unlocked on the international firmware (I recommend updating to the latest beta's) or you can continue on with the guide to return to T-Mobile firmware while maintaining SIM unlock (which will lead to boot loader unlock)...
Step 4: Convert back to T-Mobile Device: Thanks to @Dark Nightmare for this thread. Now use the MSM Tool to revert back to the original T-Mobile firmware from here: [TOOL] T-Mobile OnePlus 6T MSMDownloadTool [Firmware 9.0 v1 & v2] This will RE-LOCK your bootloader but not the SIM. You only need to do this if you want to keep the T-Mobile firmware for the updated RCS app, etc. Once the bootloader is re-locked it will take 7 days to get the unlock token shown in the next step. There will be a few security updates once you flash the original firmware back. I would update before unlocking bootloader.
Step 5: Legit Unlock Bootloader Thank OnePlus for this NOTE: Take all OTA's before unlocking. Once unlocked you will no longer receive OTA Updates. You are now on T-Mobile firmware and SIM unlocked. You can proceed to unlock the bootloader the legit way now. Simply follow the steps here: How to unlock bootloader for OnePlus 6T (T-Mobile edition). NOTE: It does take an entire 7 days for the unlock token.
Thats it, your done! You are now fully unlocked and running t-Mobile version of the software (or moved on to other ROMS all together). You can now have root on the latest T-Mobile ROM with RCS capabilities (after waiting the 7 days for the token).
Feel free to discuss all things T-Mobile in this thread.
Big thanks to the following people for posting all original work: @AnonymousTipster, @mauronofrio, @nika_bego, @Dark Nightmare