Once carriers accept the message, CLiX no longer controls its delivery outcome.

At that point, the upstream carrier assumes responsibility for delivering the SMS and returning delivery receipt data to CLiX.

Generally, any message showing sent, delivered, or undelivered status will incur charges. 

Messages rejected by upstream carriers (such as those with invalid numbers) are not billed.

For further information please refer to these articles 

What does my message status indicate?

How to troubleshoot SMS Delivery Issues ?