Mensagem original de: Chris Green ,
If you're using Sprint or Verizon (Or any other CDMA Network) and bought the phone used, then there is a chance that the device has a bad ESN number. It means that the phone is "blacklisted" and the network will not allow it to connect, because it has been reported as stolen, or it's original owner had canceled the network plan without paying the cancellation fee. [http://forums.androidcentral.com/general-help-how/27490-what-does-clear-esn-bad-esn-mean-cdma-phone.html|Here] is a thread that explains the full meaning of this. If the phone doesn't have a bad ESN, then do as rab777hp said, and check your carrier settings / obtain a new SIM card.