Go to Settings
Tap Sounds & Haptics
Under Ringtone and Alerts, drag the volume slider to the right
Turn off Change with Buttons if you want the ringtone volume to stay fixed
Make sure Silent Mode is off using the Ring/Silent switch
Increase the volume using the volume up button while no media is playing
Check that Do Not Disturb or Focus mode is not silencing calls
Try a different ringtone in Settings > Sounds & Haptics > Ringtone
Clean the speaker grille if sound is low
Restart the iPhone
Update iOS to the latest version
Reset all settings if the issue continues
Contact Apple Support if the ringtone is still too quiet