US Cellular just launched an "iPhone 8 for $0" offer with no trade-in required. The carrier also has discounts for the ($10 a month for 30 months), ($3 a month for 30 months) and LG G7 ThinQ ($300 off).
But there's really no such thing as a free, which is why these types of deals never use "free," instead saying "for $0."
Under the terms of the deal, one of the conditions that stands out is that it requires you subscribe to US Cellular's Device Protection Plus, which costs $12 a month -- over the 30 months required, that adds up to $360. Which means if you didn't want the service, that's what you're paying for that $0 iPhone.
This isn't an unusual practice and it's still a deal, I'm just saying you should make a habit of looking free gift horses in the mouth.