Since 2015-08-21 16:52:00 UTC AT&T USA have been experiencing an issue with their SMS service.
As a result of this issue, MO SMS messages and standard rate MT SMS messages are not being received by subscribers on the affected carrier.
MO SMS messages are being queued by the carrier and MT SMS messages are being queued on the SMS gateway for delivery once this issue has been resolved.
The carrier are aware of this issue and are working to resolve it as soon as possible. There is currently no estimated time to resolution, however we will issue an update as soon as more information becomes available.