hey i just recently bought this iphone off craigslist and everything seemed fine until i noticed my friends saying that i wasnt answering my phone calls, but my phone wasn't ringing at all so i couldnt pick it up. i tried calling my iphone while it was asleep and it still didnt take the call. i googled the issue and they said to rename an iphone file called power management or something but i forgot anyways that fixed the issue but the battery life sucks. so i'm thinking about grabbing another iphone through the mail service but its unlocked, but i do have access to an att sim. so the thing i was wondering is what do they ask for when you call into the iphone support asking for a mail replacement, not the advance replacement where they put a hold on your account. i would like to avoid this but there really isnt any other way to fix this problem because it seems like its happening to a lot of iphones.

Details
Unlocked 3g iphone with 3.0 firmware running on tmobile.