Open Settings
Tap Sounds & Haptics
Under Ringer and Alerts, drag the volume slider to the right
If needed, turn off Change with Buttons to keep the ringer volume fixed
Use the volume up button on the side of the iPhone to raise the ringer if Change with Buttons is enabled
Make sure Silent Mode is off using the Ring/Silent switch on the side of the iPhone
Check that Focus or Do Not Disturb is not silencing calls
Test the ringer by calling the iPhone or using a ringtone in Sounds & Haptics