I have been using a sprint BB world phone for ages. I travel a lot for work, and sprint sent me a SIM card so my phone would work outside the USA. I want to know why the sim won't work when I return to the USA.... I have tried placing it in numerous phones, and have tried using it in my sprint BB at home, but no go. Today I went to a sprint store, and the tech there told me I need to unlock the SIM card!!! My phone is unlocked. He told me to contact sprint customer service. After 1 1/2 hours on the phone, and being transferred 4 times, I am hoping someone here can help.