[OnePlus 6T  Guides, News, & Discussion] :
It doesn't get easier than this:

Step 1: Convert device to international ROM and unlock the bootloader. Thanks to [MENTION=9557721]AnonymousTipster[/MENTION] 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.

Question: where / how do I 'issue the command 'fastboot oem unlock' ?

Am I leaving my 6T plugged into the computer after running the MSM progam?

Please Clarify for us with less experience,

Thanks![/QUOTE]

Are u serious.. read this:

https://xdaforums.com/oneplus-6t/how-to/t-mobile-6t-to-international-t3888307/post79561184

There is no loophole if u follow my directions

Most of the good stuff

Betas have always had ota available; Starting with oxygen os 9.0.12 stable ota for tmo converts works fine.

4. When u convert your fajita u use a patched msm tool that works around the simlock allowing u to bypass it to unlock bootloader. Once u do so, if you use msm 9.0.11 u can then turn off the simlock, by sending 2 commands in bootloader mode aka fastbooot mode..fastboot erase modemst1 then fastboot erase modemst2. Now, those commands have now permanently shut off the T-Mobile simlock. So In a separate thread use the very specific T-Mobile msm tool that reverts your fajita back to the exact state it was the day u got it from tmobile..and it relocks your bootloader..but the only difference is once it boots up and u go to dev options and see Oem unlock, it will not be greyed out as it was before flashing to international version and unlocking your bootloader and turning off simlock. So at this point u are simunlocked and can follow the instructions and enable oem unlock and then reboot bootloader and fastboot oem get_unlock_code and once u have that imei your imei u can fill out the unlock bin request form on the OnePlus website, which a simple Google search will show u it. Then after an EXACT 7 days u get your unlock bin and flash it so u are totally unlocked and can unlock your bootloader again, the legit way and run tmo software rooted or whatever.. this is my second fajita, first time I got it unlocked legit way by tmo reps and waited a week etc. The second one I used the quick convert, then erased my simlock and waited for the unlock bin. But I also switched back to intl. after filling out the form and ran roms and whatever till I got the email a week later. Then u run tmo msm again, enable oem unlock, the flash the unlock bin. The simlock will not turn back on unless u saved the modemst1 and modemst2 via a DD copy and then u reflash them. I tested this as noted in various threads. But u honestly wont need it ever again. It's worth noting after u disable said simlock by erasing those 2 partitions, and then reflash tmo msm, u can then launch the tmo unlock app and it says your phone is unlocked...so I hope this wraps up your questions. I am pretty well versed in this as I have done it probably 10+ times now with many other peoples tmo fajitas. And as many times as u lock and relock wipe and re msm, nothing is ever simlocked.. u also get dual SIM capabilities once u convert.. the 3 tools u need are listed in this guides section. Tmo msm, I just linked I think a few pages back or somewhere nearby.. the msm 9.0.11, and the patched factory.exe(use with 9.0.11 first) listed in anoymous tipsters thread(u have to unlock bootloader with this one first so u can erase simlock bc they patched bootloader unlock without a simunlock on msm 9.0.12 and on but it can be bypassed once u erase simlock as stated above.. ..and lastly msm 9.0.13. (latest) once u download that patched exe and use it in 9.0.11, u can just copy the factory_patched.exe to every new msm tool directory to come out and use it within that new file.

Oh and don't forget your Qualcomm drivers. My HP omen downloaded it by itself but they're listed in the tmo msm thread And any msm thread. U use msm tool with your phone unplugged and totally off. U start the program, not as admin just double click. Then hold both volume buttons down on your phone for a 4 count, then plug in to computer. The phone doesn't turn on, it will make the connected sound if qualcomm drivers are installed correctly.then msm will read connected in blue. Hit start. Let it run till it says download complete and phone boots up. Do not interrupt. If it fails, try again. But u gotta force your phone back into bootloader /fastboot mode by holding all buttons down. Cycle through to power off, then repeat the msm process with holding buttons down on phone from a unplugged powered off state. There. It can't be described any easier man.goodluck