IPhone can receive iMessage sent by others, but what about SMS?

You enter your brother's number in the message sending interface, and then enter something casually. See if the send button is green or blue. If it is blue, it proves that it was sent by imessage, and it will only be sent as a short message if it is unsuccessful. If it is green, press and hold the send key to see if it can turn blue. If it can turn blue, it should be no problem. Only when the network is clear can it be sent through imessage.