How do i convince my mom.......?

okay, so my parents are divorced, and right now i have sprint with my mom, and i want to switch to att with my dad...i have a 2 year plan and want to switch by the end of the month....what do i do? its like $250 to switch cuz i have to pay off my phone, and my moms not the richest.....but my dad said if i get my mom to pay, i can have an iphone, and thats what ive been wanting FOREVER! and im not old enuff to get a job, so i cant make money that wya.....any other ideas