I bought a Verizon factory unlocked HTC One M8, and when I try to use it on AT&T's network, all it tells me is that 'the sim card is not from Verizon'. It has a sim card with a question with it on the status bar. I have tried changing my APN, but to no avail. I have called Verizon, who have told me that the phone was not blacklisted or anything. Can anyone help?