[T-Mobile REVVL V+ 5G] Regzim:
I was FIIIINAALLLLY able to unlock the bootloader of my recently acquired REVVL V+ 5G T-Mobile branded phone pretty much instantaneously using MTKClient on Linux Mint 20. This has been a painful but very educational journey of weeks hacking away at this beast. Some details and disclaimers:

*I inherited the phone from my dad (who had been a T-Mobile customer for like 26 years) when he went to another carrier, but was unable to even see the "OEM unlock" option under Developer options no matter what terminal wizardry I tried - it wasn't just ghosted, I'm talking NON-EXISTENT in developer options. I had to drag him to a T-Mobile store in person so he could show his ID to prove he was the previous owner and talk to a phone rep who had to dig deep in his records. They finally found out he had 2 months left on the contract when he bailed. He paid the remainder (like $40), I reset the phones, and within 5 minutes the "OEM unlock" option was available in dev options. So this proves 2 things:

1) the status of the previous owner's account DOES have a direct bearing on the carrier/SIM unlock availability, and the SIM unlock feature has a direct bearing on the "OEM unlocking" option showing up or not. This also means if you got it from an anonymous seller (Amazon, Ebay, etc) and it's got outstanding bills or a spotty history, you're pretty much screwed. I was lucky to actually KNOW the owner and have him help. Needless to say, I bought him lunch. :)

2) T-Mobile does not give a flying crap about customer loyalty.

*FWIW, the phone was bought on a "free phone" deal, so it is way branded and locked down by T-Mobile. They offer these deals at a loss on the front end (but you end up paying for the phone over time anyway), since they know it's locked down to T-Mobile which ensures customer retention (unless you're a stubborn geek). They do NOT want you unlocking the SIM to other providers even though they're legally obligated to do so when asked, or monkeying around with the phone like this even if are paid off and own it, since this basically means they will not get a repeat customer.

*So when I got home I tried every possible combination of ADB/fastboot terminal commands and phone modes I could find online - oem unlock, flashing unlock, flashing unlock_critical, download mode, fastboot mode, fastbootd mode, recovery mode - only to receive error messages in my terminal.

*MTK client is a little bit of a pain to get installed on Linux (lots of terminal commands). I had better success with following the README file instructions in the downloaded file I got from romprovider website than from their webpage's tutorial. Ultimately I had to click on "mtk_gui" within the mtkclient folder I downloaded instead of using the traditional terminal commands to start it.

*Once the GUI was running, I powered off my phone, held down both volume buttons, then plugged in the usb cable (just plugging in the cable without buttons held didn't do anything). Once the phone was detected, I navigated to the "Flash Tools" tab, clicked the "Unlock bootloader" button, and it was done within a second. After turning the phone on, it went into a bootloop about 2-3 times, then said I'd probably have to factory reset the phone. I did so, and after going through the startup process and rebooting again, I was greeted with the "Your device has been unlocked and can't be trusted" message! Yay! Neither can T-Mobile, so I'm okay with it! :cool:

Hope this breadcrumb trail can be of help to other searchers!