SMS is delivery is usually very good and very quick but it isn't 100% all of the time. Several things outside the control of the service can prevent a message from being delivered. For example,
- Messages can get delayed by mobile operators
- Phones may be switched off or in air plane mode
Mobile operators do run into problems from time to time but usually messages are queued for delivery and we'll get a status on the message when they do.
When phones are off etc. when switched back on the mobile operator will attempt to deliver the message and we'll update the status as soon as they try.
For messages that don't have a status set and the network hasn't updated the service we'll poll the messages again to determine the message status. This is automatically checked in batches in the very early morning.
One final reason for not receiving a message is that you have sent the message to the same mobile number 3 times already. If this is the case then the service triggers a flood protection. The service won't allow more than 3 messages from a campaign to be sent to that number in one day.