Call the other party's mobile phone to turn it off, send a text message and receive a receipt, and report "sending failed". What's going on here?

First, use your mobile phone to call the other party to turn it off, and send a short message receipt to report that it is unsuccessful. It is possible that the called number has been turned off.

2. If you are sure that the called number is not down and in normal state, call another phone to see if it is also down. If you make a new or fixed call, there may be something wrong with the called party.

Third, if other phones can dial, but your mobile phone can't, it may be that the called party has set up a firewall or blacklist to block you.