Why does the mobile phone prompt "Send on hold" when sending a text message to someone (not everyone)?

When sending a text message from China Mobile to China Mobile, if the other party's signal is not good and the text message has not been received for the time being, it will display that the message is on hold.

Or the direct signal is not good, and the other party will return a signal after receiving the text message, but your mobile phone did not receive it.

When China Mobile sends it to China Unicom, even if the other party receives it, the mobile phone on your side will show that the sending is on hold