What does it mean that overseas mobile phone numbers have security risks?

Please check if you can't receive enhanced information. If "enhanced information" is displayed in the SMS edit box, it means that the mobile phone is sending a short message with enhanced information.

You can enter the settings, search into the privacy space, open the information to see if you have received the SMS record. If yes, please move the required contact to the main space and try again.

If you just can't receive the SMS from Apple's mobile phone, you can receive the messages from other mobile phones. It may be because your mobile phone number was bound to an Apple device before, and the other party sent you an iMessage instead of an ordinary SMS. Please refer to Apple's official instructions to unbind the phone number from the iPhone device. The specific steps are subject to Apple's official instructions.