Messages sent to a mobile phone may be delayed for a number of reasons. The two most common are absent subscriber and mobile memory capacity exceeded.

Absent Subscriber
In the case of an absent subscriber a mobile phone may be switched off or temporarily out of coverage. If the SMSC tries to deliver a message to a mobile phone under either of these conditions then the mobile phone is noted as being absent. The SMSC will retry according to the absent subscriber retry schedule for a limited number of times just in
case the mobile phone has temporarily gone out of coverage (e.g. into a tunnel). If the mobile phone becomes available (is switched on, changes cells, receives or makes a voice call, sends a short message or the mobile phones periodic location update timer expires) then the SMSC will be informed and the message waiting in the SMSC will get delivered. Also any new message arriving in the SMSC whilst the mobile is absent will trigger a delivery attempt and restart the retry schedule from the beginning. It should be noted that when a mobile phone goes temporarily unavailable and becomes available again in the same cell there is no notification to the SMSC that it has become available unless the phone gets turned off then on, changes cells, receives or makes a voice call, sends a short message or the mobile phones periodic location update timer expires. Hence the need for the retry schedule for absent subscriber.

Mobile Memory Capacity Exceeded
In the case of memory capacity exceeded, the SMSC will continually retry to deliver the message with gradually lengthening intervals. The retry will continue for the validity period of the message.

Other Message Delays
Other less common errors encountered (e.g. Unidentified Subscriber) are subject to different retry schedules according to the error condition. Some messages will retry a few times and will then be purged because by retaining them at the head of the queue would result in other messages which could be delivered expiring before delivery can take
place.

Duplicate Messages
Duplication of messages can occur if the sending entity fails to receive an acknowledgement that a message has been successfully received by the SMSC and the message is re-submitted