Open Settings
Tap Sounds & Haptics
Under Ringer and Alerts, drag the volume slider to the right
Turn off Change with Buttons if you want the side buttons to control only media volume
Press the volume up button on the side of the iPhone
Make sure Silent Mode is turned off
Check that Do Not Disturb or Focus mode is not silencing calls
Go to Settings
Tap Accessibility
Tap Touch
Turn off Phone Noise Cancellation if needed
Disconnect Bluetooth devices if ringtone sound is going to them
Restart the iPhone if the volume still seems low