There a large number of reasons why a message may not be delivered immediately. A full list of reasons can be found in our API Reference. But in our vast experience, the top 3 reasons for a message failing are as follows:
- The mobile handset is switched off or out of coverage, and the message expires before it is delivered. This is fairly typical failure if someone goes on vacation and leaves their mobile turned off.
- The number you are sending to has been incorrectly formatted or does not exist or is a fixed line number. Check the number carefully.
- There is an issue with the network at that moment or a network is imposing a curfew (typically at night), and if this consistently happen then you should contact support.