By completely unlocked, I mean, T-Mobile will tell you that once you pay off your device, you can unlock your phone. They will tell you that it is yours, and you can do with it what you like. That seems to be a lie.
While they will cellular unlock your device, meaning you can go to AT&T or Verizon, for example, the OEM option will still be grayed out within the developer options. To view your developer options, go to your phone settings, tap on “about phone” and then rapidly tap on “build number”. The option to OEM unlock will remain grayed out (not available) and that means you cannot implement better security and privacy, by installing a free open-source ROM, for example, GrapheneOS.
That means your phone is not really your phone, because you got it through T-Mobile.