Why can Apple 4 receive text messages without inserting a calling card?

1, Apple mobile phone can be used normally after activation.

2. There is only one case of receiving a mobile phone message without inserting a calling card: the received message is an iMessage between ios settings, and it is a message sent through the network. You can communicate with each other as long as you have an Apple ID.

3, Settings → Information, open iMessage to use this function.