Hello soc08,
Thank you for using Apple Support Communities. It's our understanding that the ringer volume on your iPhone is lower than expected. We're happy to help.
It sounds like you may have Attention Aware enabled on your iPhone. With this feature the iPhone will check for your attention. If you're looking at the device It will lower the sound of some alerts. If you'd like to disable this feature go to Settings > Face ID & Passcode > Attention Aware Features. Take a look at the following resource for more information: About Attention Aware features on your iPhone X or iPad Pro
Let us know if this helps. Please include any other troubleshooting steps that you may have tried.
Have a nice day!