Volume of buttons when dialog a number
It appears that the sound volumes are closely related, but you can change the tones you have selected (at least for text messages and alerts).

You can also make sure that you are adjusting the ringer/alert volume and not the media volume.

Navigate to Settings Then open Sounds &amp; Haptics Then  under RINGER AND ALERTS enable "Change with Buttons"

Once you've done this, you can increase the ringer/alert volume from the side buttons. This alert/ringer volume is also the one that controls the the sound of the keypad. (I confirmed on an iPhone 11)

You can also confirm that your "Text Tone" is the alert you want. 

An alternate troubleshooting option would be to switch the "Ringtone" and "Text Tone" pattern to see if it is an issue with the alert tone you have selected.