I disagree. Your 'not possible' follows from out of the blue.
This refund would be indeed not possible provided that the no-refund rule was explicitly written in the AT&T policy and this policy was made clear for the client at the time he/she subscribed for this plan.
Since none of this is in place, general practice rules apply. That is, if the service had not been provided, the advance payment for this service must be refunded.