What is stopping me?

I see a Samsung FE promotion with Verizon where I could get a free phone via BIC. What is stopping me from porting that line out of Verizon and using T-Mobile’s Carrier freedom promo to get it 100% free contract free? I also see free iPhone SE2020, and other various promos. AT&T has some deals going on as well…

Is there any catch? I previously used carrier freedom when I Ported 10 lines to T-Mobile. Can I use it again for contract buyout?