Let's read the fine print. "After 18 mos you must return device in good condition or exercise purchase option." Also, "All of your payments count toward the phone cost, and if you want to keep your smartphone, simply pay the remaining JUMP! On Demand payments and residual purchase amount." Hey John Legere, what's the "residual purchase amount"?
I bought my Galaxy S4 using this program, and it was super straightforward at that time. I could either buy it outright for $600, or just pay the tax up front, then $25/month for the next 24 months (which adds up to $600). At any time I could pay off whatever balance remained on the phone.
I'm surprised to see this 18 month language you're mentioning, but I'm sure that the "residual purchase amount" is just whatever remains of the total price of outright purchasing the phone that you haven't chipped away with your monthly payments.