The hangupCause parameter is used to inform you why an outbound call did not go through. This is one of the parameters sent to your callback Url and events Url (if provided). Here's a sample of what that looks like.
Below are the situations where this might happen.
If the call goes unanswered
If the call is rejected
If the recipient's phone is off or unreachable
If you need any more clarification reach us on email@example.com.