1) When you buy a 'no sim' phone from Apple, does that mean it can be used on any network, GSM or CDMA, and you can switch when you want from one to another? 2) If you buy a used phone that is locked to a network but paid in full, can you unlock it to use on a different network? 3) Once a Sprint phone is unlocked, it can be used on any GSM network, correct? 4) Has anyone used or tried US Mobile? Thank you in advance for answering.