Verifying imessage number. How come?

Under normal circumstances, the system will automatically verify in the background.

IMessage is an instant messaging software launched by Apple, which can not only send short messages, but also send videos, address books, location information, photos and so on. The most commendable thing is that the security of iMessage is so high that it is said that it is difficult for even the FBI to decrypt the information in Message. Open the main screen of Apple's mobile phone, then click Settings-Info -iMessage, and drag the slider to the right, which is blue. Touch the receiving method to set the contact address of the iPhone and enter the correct AppleID. After opening the iMessage service, enter the SMS interface and enter the SMS content. If the other party also turns on iMessage, the phone will automatically switch to iMessage mode to send, and the content will be displayed in blue.