How do you let your parents know you need a cell phone?

so my parents are like cavemen and believe i don't need a cell phone even though i am in high shcool. I have tried all of the arguments you could think of (social aspect, saftey aspect, even everyone has one). the deal is when i get one i have to pay the bill. i get one before college. how do i ask them for one now and tell them i want/need one now and i can't wait. please help :)