Below are some reasons why the SMS might not be delivered.
The location of the sender or recipient can create a delay in text message delivery. This is most likely to happen if messages arrive all at once. If a mobile device is located outside a network's coverage area or in a spot where the network signal is blocked such as in a mountainous region, a delay in transmission may occur. In urban areas, tall buildings can also cause transmission issues. Being in an old building with poor cell reception is another possible culprit.
Mobile Device Issues
Problems with a mobile device may cause delays in text message transmission. The most obvious cause is a device that has been turned off, but a weak or uncharged battery may also negatively impact message delivery.
A text sender and receiver using different networks may have a greater chance of experiencing texting delays than those using the same network, because of communication between networks or the carrier prioritizing their own traffic.
Texting during periods of heavy network use impacts text delivery speed. Periods of increased traffic may create delay arrival of messages to the handset.
It should also be noted that DLRs are not always a good measure of when the SMS was delivered. Delivery receipts are often prioritized lower than other traffic (i.e. lower than the SMS traffic) on networks.
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