We still see a high error rate in delivering SMS messages to AT&T mobile numbers.
We are working on a few tracks: we are escalating though our SMS gateway provider to AT&T and are bringing technical changes to the SMS structure in order to improve deliverability.
In the meantime, if your hosts report this issue, we would recommend that they report the issue to AT&T via this link: https://www.att.com/contactus/index.html
. We will post updates on this page.