Recently learned that "unlocked phone" ≠ ”unlocked bootloader". Purchased an older OnePlus model that was listed as "Unlocked T-Mobile". Had to submit fastboot unlock key and other data to an online tool, got no response. Contacted support with the same info, they responded after several days saying "the unlock code you sent us is invalid" and "this is an older device and we have no way of unlocking it". Such reaponsibility from manufacturer.
FYI flashing a (hacky) Global ROM (in place of original T-Mobile/Metro/whatever) using official MSM tool allows fastboot unlocking without a code.