When an SMS message is sent to a recipient, it is expected it will be delivered to the handset within seconds. However, delivering an SMS is a relatively complex process, and an SMSC takes several measures to ensure a reliable service including retrying a message should it fail to deliver. An SMS may fail to deliver to a handset on its first delivery attempt for many reasons. These reasons can either be temporary or permanent, and if the latter, no additional retries will be attempted, and the delivery notification will be sent back to the sender to confirm the message failed.
