using iPhone 11
How do I turn on the phone ringer on my iPhone 11?
iPhone 11 Pro, iOS 15
Question marked as
Some things to check:
The ring/silent switch on the left side of the phone. See: How to put your iPhone on vibrate, ring, or silent mode - Apple Support
Settings > Phone > Silence Unknown Callers: Detect and block spam phone calls - Apple Support
Do Not Disturb: Use Do Not Disturb on your iPhone, iPad, and iPod touch - Apple Support
If none of the above, also try restarting: Restart your iPhone - Apple Support