Customer service representatives all agreed that the failure was caused by a "high number of simultaneous activations".
Many annoyed customers received the message "your activation requires additional time to complete" when trying to activate and were told they would receive an email from AT&T when they could complete the activation.
New customers cannot use any of the iPhone's functions before activating so many were left with a $600 paperweight. According to posts on Apple's support forums, some users have been waiting 15 or more hours without any resolution from Apple or AT&T.
There have also been complaints about automatically transferring an old number while others have had success moving from individual accounts to a Family Plan.
Source:
MacNN