In November, we noticed that SMS notifications sent to AT&T mobile numbers were not being delivered because of filters put in place by AT&T. (This issue did not affect SMS sent to other carriers).
We escalated through our SMS gateway provider to AT&T and brought technical changes to the SMS structure in order to improve deliverability. Following these actions, we have observed that deliverability rates have returned to normal levels for AT&T numbers for the last 3 weeks and are thus closing this issue.
Even if this issue was a top priority for the team, complete resolution took a few weeks as we were dependent on technical decisions from AT&T. Please contact us at firstname.lastname@example.org
if you have any questions. Thank you.