Why did China Unicom suddenly find that it sent out the message of balance inquiry many times, but it couldn't be sent out?

Why did China Unicom suddenly find that it sent out the message of balance inquiry many times, but it couldn't be sent out? Because of the problem of setting the number of SMS center, it is recommended to check whether the setting is correct; 2. The system is busy during the peak period of holiday SMS sending, so it is suggested to try again later or avoid the peak period. 3. The sending format is incorrect. If you send a short message to PHS users, you need to add the area code before the number; 4. The SMS inbox and sending box are full; 5. Query whether the SMS function is turned on. If the above situation has not been solved, it is suggested that you can contact the local manual customer service for handling.