First of all, how horrible are your parents that they require that you pay for your own cell phone. What cruelty!! In case you don't get I'm being VERY sarcastic.
A cell phone is not a right. Many parents require that their children pay for their phones.
As for getting emancipated, not all states allow for emancipation. Those that do, require the child show they are capable of caring for themselves without any help or supervision. So you would need to prepare a case to prove that.