Why can I not hear ringtones on iPhone 11
why Does my iPhone 11 not ring when a call is coming in?
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Some things to check:
The ring/silent switch on the left side of the phone: 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