Please leave a message when you answer the phone.

Hello, dear. Hello, the number you dialed is busy, and he will be notified if there is an incoming message. Goodbye, in this case, this person should have set Do Not Disturb, he may be busy. When someone called, he set up a short message notification to avoid missing the message. It is also possible that he doesn't like being disturbed by other people's phone calls, so he sets his mobile phone uniformly so that anyone who calls can receive SMS notifications.