When a phone is turned off or out of range the message may initially bounce back. We will re attempt to send the message to the handset for up to 12 hours.
When looking at your reports, it is important to keep this in mind. A campaign for example may initially report 5% failure rate, however 6 hours later this failure rate may have reduced by 2%. This could be explained by the message retrying within the 12 hour time period and successfully being able to reach handsets that are now turned on or within range.