"The subscriber you dialed is temporarily unable to answer your phone, so hang up directly. The subscriber you dialed will be notified by SMS, please hang up. " What does this mean?

There may be no signal on the other party's mobile phone. The system will send a text message to inform the other party that you called him at a certain time. (The other party's mobile phone has opened this service, and the SMS sent by the operator's server has nothing to do with you. (Nothing else, you can hang up. Even if you don't hang up, you will hang up automatically.