Haven't BOGOs been in the form of bill credits? For example, you buy one and get one free as long as the second phone is a new line (can't upgrade two existing lines and get BOGO). The free phone shows up on your bill in the form of the monthly equipment installation plan cost with the same amount as a bill credit. But if you leave the carrier before the 24- or 30-month agreement is up, you lose the bill credits (meaning you will owe the outstanding amount).
I haven't come across a BOGO offer that makes sense for people with an existing plan who don't want to add a new line.
T-Mobile didn’t require bill credits for the S7, S8, or Note 8. They issue a prepaid debit card for the price of the “free” phone. They do require an added line which you can cancel after they issue the debit card.