Does Sprint or T-Mobile have a plan like this if so what is it called and how could I do it?

So I've been asking my mom for the iPhone 7 plus because my iPhone s6 broke last year and I've been phoneless for a year but I've been looking for a plan where I could get the iPhone 7 plus ( 128 gigs my last iPhone was 64 and I don't want to go lower then that because I use a lot of space ) I was wondering if my I could get on a plan or something like that where we could pay off the iPhone monthly ( like 87 bucks or so ) and once we pay it off we could pay the normal iPhone bill which is like $32 I believe i hope you understand what I was trying to say thank you

T-mobile does. When you buy the phone they'll ask if you want to pay in installments or in full. Choose installments.

Pretty much all mobile providers have this plan. It's better to pay it off right away so you can switch carriers whenever you like. If you pay in installments you'll be stuck with that carrier for at least 24 months.