Is it free for Apple phones to send messages to each other?

The wrong SMS sent by the mobile phone card is charged according to the package used, regardless of what package is used. If the one on the iPhone is not called SMS, it is called iMessage.

You need to open iMessage in Settings-Info.

Then in the cellular data or WiFi environment, they can send each other.

The standard is that the send button turns blue.

Is iMessage.