The free cell phone model you get just depends on which cellular service provider you choose.  Each provider has their own selection of free phone models that you get to choose from. If you do not like any of the free models in most cases you can get the model of your choice at a huge discount if you sign up for service. You will usually have to sign a contract of at least one year in order to receive your free or discounted phone.

One of the reasons cellular service providers can offer you a high quality free cell phone is that they know their cost will be recouped  through your monthly cellular phone service fees. They gain a new customer and you get a cool new cell phone for free.  It's a win-win situation for all.