This feature will prevent a person from transferring your phone number to another SIM. This is important since many people rely on their cellular for for SMS verification services. If a bad actor was to move your number to a SIM they have in the middle of the night, they can lock out many accounts before you even wake up. Number locking will prevent this from happening. To prevent social engineering, porting your number to another carrier or SIM can not be done over the phone. Also, in order to have number lock turned on, you must have 2FA active, and an email address. It would be recommended that you use a unique email address just for this service. This will prevent a bad actor from getting your code even if they somehow hacked your public email address.
In order to unlock your number, you must log into the control panel. If you do not have access to your phone, then you would have to have the code emailed to you. This is just like any other email 2FA process. However, for this to happen, you must enter you security code. This was setup when you created your account. The system will not give any clues on where the 2FA code is going to. Once you log into your account, you would be able to go to the appropriate number you wish to transfer, and unlock that number. You will receive an SMS and email message stating that your number will be unlocked in 30 minutes. If a bad actor somehow got the access they need, there will still be a chance for you to stop everything from happening.
Once the number is unlocked, the customer can transfer the number to another SIM, device, or carrier. If porting the number out of PCS 1, then you would use your security code as the port out PIN. Again, customer service can not unlock your number. This will apply to either chat, or phone.