It is because the other party has turned on the function of full-time pass or caller alert,
When the other party's cell phone is not in the service area, or turn off the state will hear this service language.
The service message means that the other party can't receive the call,
but the information of the call made to the other party will be received when the other party's cell phone has a signal or is turned on. It is a mobile service that does not delay communication.