While it would be very difficult to provide an exhaustive list of reasons for a delay, as some reasons are technically complex and are not always explained by the operators, we can outline some of the reasons that you might see occasionally affecting your sends.
Handset temporarily unavailable, for example, due to loss of signal
Often the leading cause of delays in message delivery due to the handset not always being readily available. This is particularly true when travelling in more remote areas or when the network signal isn’t strong enough.
Handset is assigned to a new base station
Handsets and Short Message Service Centres (SMSCs) constantly ping each other for the SMSC to determine the handset status or vice versa. As part of its operation, the SMSC needs to determine where the handset’s closest base station is. During times of switching from one base station to another, message delays can occur.
SMSC’s and base stations have their own retry schedules when attempting to deliver messages
Depending on the volume and activity load of the base station, certain messages might take longer to be delivered than others. There is no prioritisation on delivering messages, but rather how often the message has been retried. The more often a message is retried, the longer the time interval is before the next retry is attempted – until the message ultimately expires.
SMSC’s and Base station reliability
If a Base station fails for any particular reason, SMSC’s automatically reroute all messages through the next closest base station. This results in the surrounding base stations increasing activity and volume that their internal logic isn’t prepared for, resulting in considerable delays in message delivery for that area. This scenario rarely occurs.