In many cases you have to get an authorization code from the carrier that sold the device in order to unlock the bootloader. That may or may not involve retrieving a code from your device, and it may or may not also involve interacting with the OEM. It depends on the details negotiated between the carrier and the OEM.

For example, T-Mobile sells devices that are both bootloader and network locked but (for some devices) provides a process to unlock both of those once certain criteria have been met (length of device ownership, account standing, etc). To be perfectly clear, for devices sold by T-Mobile they generally have to authorize you somehow before the OEM will send you a bootloader unlock code.