Sprint. You will need to get a new phone and sign a new 2-year commitment. Many cell phone providers require a commitment to help defray the cost of providing phones at a substantial discount.
It seems that now is an appropriate time to re-evaluate your cell phone needs as there are a variety of great deals out there and even some great no-contract service providers. To find the best cell phone plans to fit your needs, visit http://www.cell-phone-plans-comparisons.com where you can upgrade with your current provider or shop for new phones and service plans.