Apple phones can't answer and send text messages. What is the reason?

Apple's mobile phone suddenly can't send and receive text messages. There may be the following situations. ?

1. If the other party also opens Imessage, the short message will be sent from the network channel, which is similar in nature to WeChat. If the other party doesn't open imessage (or he's not an iphone), the SMS will be sent as a normal SMS. So check if the Imessage function is turned on.

2. It may be the operator's network. Try another place or time.

3. There may be something wrong with the mobile phone settings. Enter "*#5005*7672#" in the dial, and click "Call" to see the SMS center number set by iPhone5. Then enter "*5005*7672* SMS Center Number #" in the "Dialing Keyboard" and click "Call" to complete the setting of SMS Center Number (for example, Shenzhen Mobile * 5005 * 7672 *+861380075500 #).