Why can a mobile phone receive a message that it didn't receive when it was turned off? What is the mechanism?

The function of the mobile phone itself, when the mobile phone is turned off, short messages can enter; Even if the other party's mobile phone number is hacked, the other party's short message can still enter, and there is no need to set information to stop it. This has to start with the 370 plane incident. A unit neighbor, her husband is in the air force of 1000 kilometers. That day, I went upstairs and passed by her house. She was confused with a mobile phone in her hand. Please help me analyze it. I have called him many times every day for more than a week, but I can't get through every time. Is something wrong? I thought about it, you try texting, and she told me happily the next day! The phone is connected. He accidentally blacklisted my number while playing with his mobile phone.