Flip the Ring/Silent switch on the left side of the iPhone so the orange is not showing
Open Control Center and make sure Silent Mode is off
Press the Volume Up button to raise the ringer volume
Go to Settings
Tap Sounds & Haptics
Under Ringtone and Alert Volume, drag the slider to the right
Make sure Change with Buttons is turned on if you want to use the side volume buttons
Check Focus modes and turn off Do Not Disturb if it is enabled