What does this message mean when Apple calls? However, there is no hint in the outgoing SMS.

Flash message or level 0 message, full-screen display will not be saved in the mobile phone. Turning on this function will display the other party's number on your mobile phone.

You can do the following:

Install the SIM card you are currently using on the iPhone.

Make sure the iPhone is connected to the cellular network.

Open the Settings app, and then click Info.

In the information panel, turn on the iMessage switch. When the other party also opens iMessage, the message sent is blue.

There should be a temporary error in the mobile phone system. Try again.