How to convince my parents to let me get the sidekick 4g?

I really want it an my birthday isn't til 4 months but whenever my parents buy me something like a phone they buy it like 5 months early. Last year I got the Iphone 3g not the 4g and plus i only got it to show off to my friends an people at school because I didn't get it with contract an I couldn't text til now which is the text free with voice app an it needs internet an i don't have internet on it so I can only text at home. I asked my big brother whose the one that noes everything bout phones an stuff an my parents usually tell him to buy me something an i asked him for a NEW PHONE (i didnt say which one) an he said when i was your age i barely had a laptop (so he meant no :/) (an btw im 15 an hes 22) SO HOW CAN I CONVINCE MY PARENTS TO LET ME GET A NEW PHONE??