So here's the story, i bought a HTC one M8 AT&T version on CL. I had the phone for a few days during which time i have unlocked bootloader, rooted and S-Off the phone. Well the douche i bought the phone from must have reported it lost or stolen because its blacklisted. I've searched google and numerous forums regarding a solution so i can continue to use this phone on either the same carrier or another. I know most here wont engage in this discussion in detail as it violates forum policy, BUT if ANYONE has a legit solution that i can go with PLEASE inbox me so im not stuck with a very expensive brick.
You gotta call AT&T Customer Service from another phone, you'll get the automated service...
enter your phone number, they'll ask if your calling about your suspended line, or something of that sort... dial whatever for yes, then they'll ask if you want to re activate the sim for the original device, again dial whatever for yes, then they'll redirect you to an operator, when the phone starts ringing just hang up... then turn airplane mode on then off, and it should start working again.
Happened to me with the M7, it was reported stolen... so i had to go through this procedure 2,3,4 times a day, sometimes it would stay connected for weeks if i got lucky. But yea, that was my solution, hopefully it works for your M8, and if it does then it probably works for any phone thats blacklisted on AT&T... i wonder if anyone knows about this