Below you can find the details of the reasons mentioned when the sms is failed.

Cause Explanation:

ABSENT_SUBSCRIBER: 

When the service provider fails to reach the member/subscriber, this value is passed.

UNKNOWN_SUBSCRIBER: 

Unknown/invalid number.

BLOCKED_MASK: 

Mask is blocked by SMS Provider.

SYSTEM_FAILURE: 

Failure due to a problem in the Operator’s systems (Originating or Destination Operator).

CALL_BARRED: 

Subscriber has some kind of call barring active on the number due to which messages from unknown sources are blocked.

SERVICE_DOWN: 

Operator service is temporarily is down.

OTHER:

Message that are sent but could not be delivered for reasons that don’t fall under any mentioned category.

Numbers is not matching to the group of CLASSPRO :

If you will try to send the custom SMS via community route. Hence the message failed. You can use the promotional route for this or can ask the user to join the Classpro community GROUP.