Apple 5 mobile phone can't receive and send text messages, and it shows that it hasn't been delivered when it is sent. What's the matter?

Apple 5 mobile phone can't receive and send short messages, and when it is sent, it shows that it hasn't been delivered, so Imessage may be turned on.

1. Imessage will change automatically according to the object of your message. . Two situations. 1. If the object is also enabled with Imessage, then your short message will be sent out from the network channel. Similar in nature to WeChat

2. If the object doesn't turn on imessage (or he's not an iphone), then your short message will go out in the form of ordinary short message, taking the channel of the telecom company.