Both parties are Apple phones. They used to send messages by imessage, but they can't send messages by imessage today.

Imessage is contacted through an Apple account. For example, both parties need to log in to Apple ID, enable IMESSAGE, and both are networked. Because imessage is like an email, when the other party receives it, the message sent by itself will be displayed as sent below, and when the other party has read it, it will become read and connected to the Internet. So when the other party is not connected to the Internet, imessage will automatically switch to ordinary SMS.