How to turn ringer on,iPhone max
How to turn the ringer on
iPhone 11 Pro Max, iOS 13
Question marked as
The switch on the left side. See: How to put your iPhone on vibrate, ring, or silent mode - Apple Support
If that is on, another reason for it not ringing if someone calls that is not in your contacts is Silence Unknown Callers. See: Detect and block spam phone calls - Apple Support
Another possibility: Use Do Not Disturb on your iPhone, iPad, and iPod touch - Apple Support