Our mobile carrier for SIP2SIM have reported problems with inbound SMS failing. This looks to be affecting SMSs sent from the handset/SIM. Their engineers are investigating.
Update from carrier on Sept 1 17:36 All indications are that message levels have returned to normal.
Update from carrier 2 Sept, 10:37 - The engineers have reviewed the traffic and observed no further issues. The incident is now being closed.