The NO_ANSWER and NO_USER_RESPONSE callback parameters are triggered when the call is not answered but the circumstances are different as shown below.
NO_ANSWER: This cause is used when the called party has been alerted but does not respond with a connect indication within a prescribed period of time.
The phone rang but the called party did not pick the call.
NO_USER_RESPONSE: This cause is used when a called party does not respond to a call establishment message with either an alerting or connect indication within the prescribed period of time allocated.
The call never got to the phone of the called party. This is mainly because of an error on the provider's end.