A planned change internal to the carrier we use for much of our incoming SMS services had an unexpected and undetected side effect. This meant some incoming SMS were not processed and did not get to us. The undelivered messages were rejected, which would usually have resulted in them being resent, or the sender advised of non delivery. However, reports from customers indicate that the sender was not made aware and messages were lost. The issue was resolved Monday morning.
The issue was identified by the carrier and rectified. The carrier is improving processes and monitoring to ensure this type issue is detected much more quickly should it ever happen in future.