I'm new to the CDMA world, having been on T-Mobile my whole life. If I signed up for a RingPlus plan, bought a Sprint phone, and activated it with RingPlus, the phone still will not work until I buy a SIM card? I did this, and was confused why it wasn't working, until I realized this. Can someone explain what I need to do? Do I need a SIM, which one, etc.