I was contacted by some one that told me he used the IMEI from an older device, phone etc.. Said his carrier just needed a good IMEI number, one that is in the IMEI database, to register the new SIM card. He then put it in his 685 and it worked fine.
The IMEIs of these tablets are not in the database. So if you have or know of someone with an old phone, give it’s IMEI number to AT&T, see if they can then register a new SIM card.
