Open the Settings app
Tap Sounds & Haptics
Under Ringer and Alerts, drag the volume slider left or right
Turn off Change with Buttons if you want the side volume buttons to control media only
Use the side volume buttons to adjust ringer volume if Change with Buttons is enabled
Make sure Silent Mode is off using the Ring/Silent switch on the side of the iPhone
Check Focus or Do Not Disturb settings if calls are not sounding
Test the volume by calling your iPhone from another phone