Sometimes there is no ringtone sound or message sound. Sometimes there is. I tried switching phones and sometimes it happens on the other phone too, but not as often. Even the flash on the back doesn't come on.

Hello,

See these steps:

1. Is the side silent mute button on? Does any red show? Slide it back and fourth.
2. How about the volume buttons? Is the volume very low? Move the buttons.
3. Is Do Not Disturb on? Go to Settings > Do Not Disturb. Is it turned on? Check the times.
4. Go to Settings > Sounds & Haptics. Check the Ringer and Alerts slider.  Try changing the sound pattern for the type of alert.
5. Go to Settings > Notifications. Is the app listed?
6. Do other sounds work? What about phone calls? 
7. Do you have an Apple watch? Many of the notifications will go to the watch instead of the phone.

Note: you can send text messages to yourself for testing purposes.

Good luck.

I have the same problem on my iPhone XR and have tried every single suggestion on this site and other sites. Nothing seems to work. I'm guessing it is the old Apple 'obsolescence' revenue generating tactic coming into play seeing as my iPhone is now 2.5 years old. Exceptionally frustrating