Apple 6 sent a text message and sent it back. What happened?

Troubleshooting for Iphone6 plus not sending SMS: 1. Make sure the signal is normal and turn off the flight mode. 2. Check whether the SIM card status is normal. 3. Check whether the send key is displayed in bright color and clickable. 4. If you can't click Send after entering the text, the "Send" key is still gray. 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 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: use iPhone to dial *5005*7672*+86 SMS center number # to delete SMS center number; use iPhone to dial ##5005*7672# to query and set SMS center number; use iPhone to dial *#5005*7672# to use internal command, then use delete command to set SMS center number.