I have a call reminder service on my mobile phone. Suppose I send text messages to others. If they don't receive it, will I receive a reminder? What if they don't read it?

If you send a short message to someone, you will receive a short message saying that the other party has not received the short message when the signal is bad, there is no signal, or the phone is not turned on. If the other party's mobile phone is turned on and there is no signal, it doesn't matter whether you check the SMS or not. The SMS receipt will tell you that the SMS has been delivered.