Right, but the "phone" part of a burner phone is essentially superfluous, it's the sim card that serves the purpose here. Of course you need to put it into something but it doesn't matter what. You can swap your main sim card with a "burner" sim, get the code to authenticate, then swap back. It takes 2 minutes.
You can get a free SIM, but even if you have to pay 5$ or whatever... many people still need to use facebook messenger to get in touch with some people, for instance.
Even if you take it literally as a physical phone instead of SIM, the SMS verification works equally well with 20 EUR Nokia dumbphone as with the latest iPhone.
Yes, but there's no way I'm investing neither money nor time in jumping through silly Facebook hoops. What's to stop them from suddenly requiring dental x-rays, or some other nonsense, for "security reasons"?