We currently use a third-party provider https://www.clickatell.com/ to send SMS messages to our users on our website.
We use this service to send confirmation codes to our customers and just recently we found that a particular message sent from the system never arrives at the user's mobile. We have tried other providers such as Vodafone and Telstra and it works all the time but just not Optus. The mobile number is correct and Clickatell confirms that the message was sent to Optus.
Does Optus have some SPAM/Filtering mechanism in place that could be doing this?