Apple iPhone SMS photo shows that it hasn't been delivered yet. What happened?

IPhone SMS shows undelivered, which may be as follows:

1. Confirm that the signal is normal and close the flight mode.

2. Check whether the SIM card status is normal.

3. Check whether the send key is bright and clickable.

4. If you can't click Send after entering the text, the "Send" key is still gray. Please turn off "Show Subject Bar" in Settings > SMS >.

5. If you send text messages to multiple people and manually enter numbers, please make sure to use line breaks instead of spaces to separate each number.

6. If you can receive or not send short messages, you can edit the number of the short message center through internal commands after eliminating the above problems:

Set SMS center number: dial *5005*7672*+86 SMS center number # with iPhone;

Delete SMS center number: dial ##5005*7672# with iPhone.

Inquire about the set SMS center number: dial *#5005*7672# with iPhone.

When using the internal command, first use the delete command, and then set the SMS center number.