So my 4s locked up at least 3 times now. Twice after a full restore via iTunes. Apple agreed to send out a replacement.....but when I goto activate on Sprint website, it fails to recognize the imei number...thus, no activation. Finally I go to the carrier and see that it's Verizon. Are the phones locked to specific carriers or are they all the same phones for all 3 carriers? Oh and also, this replacement screen has a yellow tint compared to my original. Should I be concerned about that too....or does it go away after some time?