Use the Ring/Silent switch on the left side of the iPhone and move it toward the screen so the orange is not showing
Press the Volume Up button on the left side of the iPhone
Open Control Center and raise the volume slider
Go to Settings
Tap Sounds & Haptics
Under Ringer and Alerts, drag the slider to the right
Turn off Change with Buttons if you want the ringer volume to stay fixed
Make sure Do Not Disturb or Focus is turned off if calls are not sounding
Check that Bluetooth is not sending audio to another device
Restart the iPhone if the ringer still seems low