看起来很炫酷，不了解的人很可能会被吓一跳，但这真的只是一个古老的GSM SMS功能，实现这个功能并不需要通过运营商。事实上，只要你有一台可以控制短信格式的手机终端（比如root过的安卓，或者越狱后的iPhone），你自己下个App后也可以发送这种短信。而对于接收方而言，目前的大部分手机都能够接收，包括iPhone和大部分Android设备


以下转引自wiki：

Flash SMS A Flash SMS is a type of SMS that appears directly on the main screen without user interaction and is not automatically stored in the inbox. It can be useful in emergencies, such as a fire alarm or cases of confidentiality, as in delivering one-time passwords.

其实在GSM中一共定义了4种不同Class的短信类型，这种短信只是其中的第一种：

Classes identify the message's importance as well as the location where it should be stored. There are 4 message classes.
- Class 0: Indicates that this message is to be displayed on the MS immediately and a message delivery report is to be sent back to the SC. The message does not have to be saved in the MS or on the SIM card (unless selected to do so by the mobile user).
- Class 1: Indicates that this message is to be stored in the MS memory or the SIM card (depending on memory availability).
- Class 2: This message class is Phase 2 specific and carries SIM card data. The SIM card data must be successfully transferred prior to sending acknowledgement to the SC. An error message will be sent to the SC if this transmission is not possible.
- Class 3: Indicates that this message will be forwarded from the receiving entity to an external device. The delivery acknowledgement will be sent to the SC regardless of whether or not the message was forwarded to the external device.

实现这个功能，其实就是向对方发送一个Class 0的短信而已。