The OnePlus 8T is available to purchase from T-Mobile and OnePlus’ own online store, though the phone differs slightly depending on where you bought it from. The T-Mobile includes an IP68 rating and various software tweaks, and just like most phones you buy through carriers, it won’t work on other networks without being unlocked. However, the process for unlocking the T-Mobile variant seems to be broken at the moment.
OnePlus has an unlock token form on its website, where owners of carrier-branded models can enter their phone’s serial number, unlock code (provided by the carrier), and IMEI. However, there are multiple reports that the website rejects all unlock codes with the message “Please enter a valid unlock code.”
This is similar to what happened with the T-Mobile OnePlus 8 earlier this year, which also couldn’t be unlocked for a short period of time due to errors with the same tool. It’s possible OnePlus hasn’t fully updated its website for the 8T yet, or there could be communication issues between the company’s servers and T-Mobile.