RESOLVED: 2015-08-21 21:24:00 UTC 

AT&T USA have now confirmed resolution of the issue affecting their SMS service. Any MT messages that failed with response code 1020 during this period may be retried at the discretion of the content provider. 

They have also informed us that from the start of the incident until 19:47:00 UTC, MO SMS were lost within their platform without the possibility of retrieval.