A detailed explanation of all status values can be found on: How to Check SMS Logs & Status.

Unfortunately in most cases the local Mobile Network Operators (MNOs) do not return back a detailed reason if an SMS failed. The most common reason for a FAILED message is an inactive subscriber/number. 

A message on status FAILED is not per se a problem. If all or most of your messages fail or if you know for sure that a number is active but messages are failing anyhow, then please contact our NOC support following the mentioned procedure.

Please note that it is impossible to send SMS to inactive devices same as it is impossible to call a disconnected phone.

A high FAILED rate can also be the result of trying to send messages towards a destination, which requires a registration procedure first.

Related articles:

• Do you charge for non-delivered SMS
• Invalid and inactive phone numbers